C语言：在磁盘上创建一个 test.txt 文件，使用记事本打开后先写入 40 道单选题和相应的选项和正 确答案。再编写一个单项选择题标准化考试系统，该系统功能要求： A、 用文件保存试题库。（每个试题包括题干、4 个备选答案、标准答案）。 B、 试题录入、修改和删除。 C、 试题抽取：每次从试题库中可以随机抽出 N 道题（N 由键盘输入）。 D、 考生答题：用户抽取 N 道试题进行答题，键盘输入每道题的答案。 E、 自动判卷：系统可根据用户答案与标准答案的对比实现判卷并给出成绩。 系统操作过程中能够正确读取和更新试题文件，并进行正常的试题管理、抽取、答题和判卷 操作，该过程在控制台中显示，人机交互方便。

以下是C语言实现该考试系统的代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#define MAX_QUESTIONS 100
#define MAX_OPTIONS 4
#define MAX_ANSWER_LEN 2

typedef struct {
    char question[100];
    char options[MAX_OPTIONS][50];
    char answer[MAX_ANSWER_LEN];
} Question;

void init(Question *questions, int *num_questions);
void display_menu();
void add_question(Question *questions, int *num_questions);
void edit_question(Question *questions, int num_questions);
void delete_question(Question *questions, int *num_questions);
void select_questions(Question *questions, int num_questions);
void take_exam(Question *questions, int num_questions);

int main() {
    Question questions[MAX_QUESTIONS];
    int num_questions = 0;
    char choice;

    init(questions, &num_questions);
    do {
        display_menu();
        scanf(" %c", &choice);
        switch (choice) {
            case '1':
                add_question(questions, &num_questions);
                break;
            case '2':
                edit_question(questions, num_questions);
                break;
            case '3':
                delete_question(questions, &num_questions);
                break;
            case '4':
                select_questions(questions, num_questions);
                break;
            case '5':
                take_exam(questions, num_questions);
                break;
            case '6':
                printf("Goodbye!\n");
                break;
            default:
                printf("Invalid choice. Please try again.\n");
        }
    } while (choice != '6');

    return 0;
}

void init(Question *questions, int *num_questions) {
    FILE *fp = fopen("test.txt", "r");
    if (fp == NULL) {
        printf("Error opening file.\n");
        return;
    }

    char line[200];
    int i = 0;
    while (fgets(line, sizeof(line), fp) != NULL) {
        char *token = strtok(line, ",");
        strcpy(questions[i].question, token);

        for (int j = 0; j < MAX_OPTIONS; j++) {
            token = strtok(NULL, ",");
            strcpy(questions[i].options[j], token);
        }

        token = strtok(NULL, ",");
        strcpy(questions[i].answer, token);

        (*num_questions)++;
        i++;
    }

    fclose(fp);
}

void display_menu() {
    printf("\n1. Add question\n");
    printf("2. Edit question\n");
    printf("3. Delete question\n");
    printf("4. Select questions\n");
    printf("5. Take exam\n");
    printf("6. Quit\n");
    printf("Enter your choice: ");
}

void add_question(Question *questions, int *num_questions) {
    if (*num_questions >= MAX_QUESTIONS) {
        printf("Maximum number of questions reached.\n");
        return;
    }

    printf("Enter question: ");
    getchar();
    fgets(questions[*num_questions].question, sizeof(questions[*num_questions].question), stdin);

    for (int i = 0; i < MAX_OPTIONS; i++) {
        printf("Enter option %d: ", i + 1);
        fgets(questions[*num_questions].options[i], sizeof(questions[*num_questions].options[i]), stdin);
    }

    printf("Enter answer: ");
    scanf(" %s", questions[*num_questions].answer);

    (*num_questions)++;
}

void edit_question(Question *questions, int num_questions) {
    int question_num;

    printf("Enter question number to edit: ");
    scanf("%d", &question_num);

    if (question_num < 1 || question_num > num_questions) {
        printf("Invalid question number.\n");
        return;
    }

    printf("\nQuestion: %s", questions[question_num - 1].question);
    for (int i = 0; i < MAX_OPTIONS; i++) {
        printf("Option %d: %s", i + 1, questions[question_num - 1].options[i]);
    }
    printf("Answer: %s\n", questions[question_num - 1].answer);

    printf("Enter new question: ");
    getchar();
    fgets(questions[question_num - 1].question, sizeof(questions[question_num - 1].question), stdin);

    for (int i = 0; i < MAX_OPTIONS; i++) {
        printf("Enter new option %d: ", i + 1);
        fgets(questions[question_num - 1].options[i], sizeof(questions[question_num - 1].options[i]), stdin);
    }

    printf("Enter new answer: ");
    scanf(" %s", questions[question_num - 1].answer);
}

void delete_question(Question *questions, int *num_questions) {
    int question_num;

    printf("Enter question number to delete: ");
    scanf("%d", &question_num);

    if (question_num < 1 || question_num > *num_questions) {
        printf("Invalid question number.\n");
        return;
    }

    for (int i = question_num - 1; i < *num_questions - 1; i++) {
        strcpy(questions[i].question, questions[i + 1].question);
        for (int j = 0; j < MAX_OPTIONS; j++) {
            strcpy(questions[i].options[j], questions[i + 1].options[j]);
        }
        strcpy(questions[i].answer, questions[i + 1].answer);
    }

    (*num_questions)--;
}

void select_questions(Question *questions, int num_questions) {
    int num_selected;

    printf("Enter number of questions to select: ");
    scanf("%d", &num_selected);

    if (num_selected > num_questions) {
        printf("Not enough questions in database.\n");
        return;
    }

    srand(time(NULL));
    for (int i = 0; i < num_selected; i++) {
        int rand_num = rand() % num_questions;
        printf("\n%d. %s\n", i + 1, questions[rand_num].question);
        for (int j = 0; j < MAX_OPTIONS; j++) {
            printf("%c. %s", 'A' + j, questions[rand_num].options[j]);
        }
    }
}

void take_exam(Question *questions, int num_questions) {
    int num_selected, score = 0;

    printf("Enter number of questions to answer: ");
    scanf("%d", &num_selected);

    if (num_selected > num_questions) {
        printf("Not enough questions in database.\n");
        return;
    }

    srand(time(NULL));
    for (int i = 0; i < num_selected; i++) {
        int rand_num = rand() % num_questions;
        char answer[MAX_ANSWER_LEN];
        printf("\n%d. %s\n", i + 1, questions[rand_num].question);
        for (int j = 0; j < MAX_OPTIONS; j++) {
            printf("%c. %s", 'A' + j, questions[rand_num].options[j]);
        }
        printf("Enter your answer: ");
        scanf(" %s", answer);
        if (strcmp(answer, questions[rand_num].answer) == 0) {
            score++;
        }
    }

    printf("\nYou scored %d out of %d.\n", score, num_selected);
}

其中，`Question` 结构体表示一个试题，包括题干、备选答案和标准答案。`init` 函数从文件中读取试题库，并将每个试题保存在 `questions` 数组中，同时更新试题数量 `num_questions`。`display_menu` 函数显示菜单，让用户选择操作。`add_question` 函数让用户输入一个新的试题，并将其添加到 `questions` 数组中。`edit_question` 函数让用户选择要编辑的试题，并允许用户修改其内容。`delete_question` 函数让用户选择要删除的试题，并将其从 `questions` 数组中移除。`select_questions` 函数让用户输入要随机抽取的试题数量，并从 `questions` 数组中随机选择相应数量的试题进行展示。`take_exam` 函数让用户输入要回答的试题数量，并从 `questions` 数组中随机选择相应数量的试题进行考试，同时计算分数。