典型偏头痛的先兆症状可能是由于（）
A.颅外动脉扩张
B.颅外动脉收缩
C.颅内动脉收缩
D.颅内动脉扩张
E.颅内和颅外动脉扩张
参考答案：C
I hope there will be ______ cars and ______ pollution in the future, thus people will live longer. [ ]
A. more, less
B. fewer, less
C. fewer, more
D. fewer, fewer
作业中断后为什么再次进入有限空间内动火作业时必须进行气体复验？
