![](https://cos-cdn.shuashuati.com/pipixue-wap/2020-1230-1107-56/ti_inject-812ce.png)
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.