为了保证两相在塔板上的接触，板式塔塔顶必须要有（）。
A.安全阀或放空阀
B.液封
C.回流
D.再沸器
参考答案：C
Our manager said ________ couldn't finish his own task on time must do extra work on Saturday.[ ]
A. which
B. who
C. whichever
D. whoever
一级车站站房设施有哪些？
