单线双向运行区段的进站信号机应选用的组合是()。
A.LXZ、YX
B.LXZ、YX、1LXF
C.LXZ、YX、2LXF
D.LXZ、YX、LXF
参考答案:B
-The ground is wet.
-It must have rained last night,_____? [ ]
A. hasn't it
B. didn't it
C. mustn't it
D. isn't it
志愿服务的五要素是什么?