John has tried many ways to solve the physics problems, but ________ works.
| A．no one | B．anything | C．none | D．nothing |
After half a year’s training, they were made entirely used ____ underwater.
| A．stay | B．to stay | C．to staying | D．staying |
Thank you very much for the trouble you have taken _________ me with my English.
| A．helping | B．to help | C．help | D．helped |
Before the conference, a new problem came up __________ could speak on behalf of our group.
| A．that | B．who | C．which | D．how |
He dressed up and went to the party as if ___________.
| A．was invited | B．had been invited | C．invited | D．to be invited |
Scientist are not sure about the ___________ of water on the Mars.
| A．occupation | B．existence | C．demand | D．treasure |