目前，从世界上已经投入开发油田的资料来看，油气田建设总周期长达（）年
A.1－2年
B.3－4年
C.15－20年
D.6－10年
参考答案：D
— Why does the Lake smell terrible?
— Because large quantities of water _____. [ ]
A. have polluted
B. is being polluted
C. has been polluted
D. have been polluted
-It's 10 o'clock. I must go now.
-It's raining outside. Don't leave __________ it stops.[ ]
A. when
B. since
C. while
D. until
