What does the yield return statement specify in the following C# code snippet?

public System.Collections.IEnumerator GetEnumerator()
{
    foreach (char ch in chrs)
    yield return ch;
}

What is the purpose of the "using" keyword in C#?

A. To define namespaces

B. To declare variables

C. To import external libraries

D. To create instances of classes

What does the "break" keyword do in a switch statement?

A. Exits the switch statement

B. Skips the current iteration

C. Continues to the next iteration

D. Throws an exception

Which of the following is NOT a valid C# access modifier?

A. public

B. internal

C. protected

D. private

What is the purpose of the "readonly" keyword in C#?

A. To declare a constant

B. To prevent a variable from changing

C. To allow a variable to be changed

D. To declare a static variable