Which sign means “Don't eat in class.”?

A: Did you solve _ problem you were discussing at the meeting?
B: No, we didn't. It was _ difficult problem we had never met.
A. the, the B. a, a C. the, a

A: Do you like popular songs?
B: Yes , I do. _ I’m too busy to enjoy them.
A. But B. SoC. And

A: Can I help you, madam?
B: I want_ .
A . two boxes of apple
B. two glass of water
C. two bowls of porridge

A: Do you miss Mr Chen?
B: Yes. I do._ have you ever heard of him since he left?
A. By the way
B. On the way
C. in the way