The professor can hardly find sufficient grounds his argument in favor of the theory.
| A．which to base on |
| B．on which he bases |
| C．to base on which |
| D．which to be based |
— The terrible accident is under investigation．
— Actually, quicker action _____ those workers trapped in the mine．
| A．might have saved | B．must have saved |
| C．should have saved | D．could have saved |
I have been living in the United States for twenty years, but seldom _____ so lonely as now．
| A．have I felt | B．I had felt | C．I have felt | D．had I felt |
In the study of modern science, there are many cases _____ soldiers can teach officers．
| A．when | B．who | C．which | D．where |
— Why didn’t they complete the task in time?
— Because it was not until late afternoon _____ they began to solve the major problem．
| A．when | B．that | C．before | D．which |
Your story is perfect; I’ve never heard _____ before．
| A．a better one | B．the better one | C．the best one | D．a good one |