___________ caused the accident is still a complete mystery.
| A．What | B．That | C．How | D．Where |
The reason for his success is ______ he has worked very hard.
| A．why | B．that | C．whether | D．how |
The weather turned out to be fine yesterday. I_____ the trouble to carry my umbrella with me.
| A．should have taken | B．could have taken | C．needn't have taken | D．mustn't have taken |
I was about to leave the office ______ an unexpected visitor came.
A. while B. as C when D. before
It was a matter of ________ would take the position.
| A．who | B．whoever | C．whom | D．whomever |
People try to avoid public transportation delays by using their own cars, and this ____ creates further problems.
| A．in turn | B．in all | C．take turns | D．by turns |