Fill in the gaps with can, can't, could, or couldn't.

3 A: I..... send this email.
   B: Don't worry, I'll help you.
4 A: ........ I use your computer, sir?
   B: I'm afraid you......
5 A: .......... you ride a bike?
   B: No, I ...... but my brother ..........
6 A: It ...... rain tomorrow.
   B: Oh, no! That means we...... have a picnic!
7 A: Why isn't Harvey in school today?
   B: I'm not sure. He .......be unwell.