创建一个名为 `Person` 的类，包含以下属性：`name`、`age`、`gender` 和 `nationality`。`nationality` 是一个类变量。创建一个方法 `change_nationality()`，用于更改人的国籍。请给出使用示例

class Person:
    nationality = 'unknown'

    def __init__(self, name, age, gender):
        self.name = name
        self.age = age
        self.gender = gender

    def change_nationality(self, nationality):
        Person.nationality = nationality

# 使用示例
person1 = Person('Tom', 30, 'male')
person2 = Person('Lily', 25, 'female')

print(person1.nationality)  # 输出：unknown
print(person2.nationality)  # 输出：unknown

person1.change_nationality('USA')
print(person1.nationality)  # 输出：USA
print(person2.nationality)  # 输出：USA

person2.change_nationality('China')
print(person1.nationality)  # 输出：China
print(person2.nationality)  # 输出：China