(1)
1 c
2 e
3 a
4 d
5 b
(2)
1. mustn't
2 doesn't have to
3 had to
4 don't have to
5 must
(3)
1. I think I'm a friend of the Earth.
2. I blow my nose with paper tissues.
3. Actually, no, I don't cross the road quickly.
4. I enjoy rereading old magazines and newspapers. It's like memories about past.
5. My favourite kind of transport is car.
6. I think it is a great threat to our world. Mankind ought to be more polite and clever.