18. It was ______ back home after the experiment.
| A．not until midnight did he go | B．until midnight that he didn’t go |
| C．not until midnight that he went | D．until midnight when he didn’t go |
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 |
After the new technique was introduced, the factory produced_____ cars in 2008 as the year before.
| A．as twice many | B．as many twice | C．twice as many | D．twice many as |
Great changes ______ in China in the past 20 years.
| A．took place | B．have taken place | C．were taken place | D．have been taken place |