用所给词的适当形式填空。

1. Our headmaster invited a singer ________ (sing) for us this morning.

2. I _______ (clean) my room at 9:00 last Sunday.

3. They can do that thing by ________ (they).

4. I think _______ (predict) the future is a bit difficult.

5. The teacher told us that light ________ (travel) much faster than sound.