After years of research, he found the key ____ differences between species.
| A．to explain | B．to explaining | C．of explaining | D．explaining |
The nurse whose name is Mary assisted me ______ the operation they day before yesterday.
| A．perform | B．to performing | C．on performing | D．in performing |
---Shall I go to the party tomorrow?
---I would rather _____.
| A．come | B．should come | C．would come | D．came |
The brave soldier, ________, jumped into the cold river to save the girl who wanted to kill herself.
| A．risked drowning | B．risking drowning |
| C．risked being drowned | D．risking to be drowning |
The old man said the accident______ his careless driving, so a lot of money _____ be paid by the driver.
| A．was due to , was due to | B．due to , was due to |
| C．is due to , was due to | D．is due to , was due to |
We are not going for the simple reason _______ we can’t afford it
| A．why | B．that | C．which | D．how |