By the year 2020 scientists _____ a lot of research on AIDS.
| A．had done | B．do | C．are doing | D．will have done |
She reached the top of the hill and stopped _____on a big rock by the side of the path.
| A．to have rested | B．resting | C．to rest | D．rest |
I can’t imagine _____that with such a famous author.
| A．work | B．to work | C．to be working | D．working |
She pretended ________the letter I wrote the day before yesterday.
| A．not to receive | B．not receiving | C．not to hear from | D．having not received |
Our father often told us in the past that _____is believing.
| A．to see | B．seeing | C．see | D．to be seen |
Is______necessary to change trains at Beijing?
| A．this | B．that | C．it | D．he |