There is no way of knowing why one man makes an important discovery ______ another man, also intelligent, fails.
| A．since | B．if | C．as | D．while |
Not until all the fish died in the river ____________ how serious the pollution was.
| A．did the villagers realize |
| B．the villagers realize |
| C．the villagers did realize |
| D．didn’t the villagers realize |
If he ___________ my advice then, he would not be in trouble now.
| A．should take | B．took |
| C．could take | D．had taken |
It doesn’t matter ___________ you turn right or left at the crossing — both roads lead to the park.
| A．whether | B．how |
| C．if | D．when |
It was in the school __________ he had studied ___________ he began the important experiment.
| A．that; where | B．in which; which |
| C．where; that | D．where; in which |
I felt somewhat sad and was about to leave _________ something happened which drew my attention.
| A．unless | B．until |
| C．when | D．while |