Create a Single Page Application (SPA) in React App with proper navigation.

To create a SPA in React, we can use React Router. Here's an example of how to create a SPA in React with proper navigation:

1. First, create a new React app using `create-react-app`.

2. Install React Router by running the following command in your terminal:

   npm install react-router-dom

3. In your `src` folder, create a new folder called `pages`. Inside this folder, create a new file for each of your pages. For example, you could create a `Home.js` file and a `About.js` file.

4. In each of your page files, create a React component that represents that page. For example:

   import React from 'react';
   
   function Home() {
     return (
       <div>
         <h1>Welcome to my website!</h1>
         <p>Here is some content.</p>
       </div>
     );
   }
   
   export default Home;

5. In your `App.js` file, import `BrowserRouter` from `react-router-dom` and wrap your entire app in it. Then, create a `Switch` component that will render the appropriate page based on the URL.

   import React from 'react';
   import { BrowserRouter, Switch, Route, Link } from 'react-router-dom';
   import Home from './pages/Home';
   import About from './pages/About';
   
   function App() {
     return (
       <BrowserRouter>
         <nav>
           <ul>
             <li>
               <Link to="/">Home</Link>
             </li>
             <li>
               <Link to="/about">About</Link>
             </li>
           </ul>
         </nav>
         <Switch>
           <Route path="/about">
             <About />
           </Route>
           <Route path="/">
             <Home />
           </Route>
         </Switch>
       </BrowserRouter>
     );
   }
   
   export default App;

6. In the example above, we're using `Link` components to create links to our pages, and we're using `Route` components to specify which component should be rendered for each URL. The `Switch` component ensures that only one page is rendered at a time.

7. Start your app by running `npm start` in your terminal. You should see your navigation links and be able to navigate between your pages.