水库遇下游保护对象的设计洪水时在坝前达到的最高水位称( )。
A．校核洪水位
B．兴利水位
C．防洪限制水位
D．防洪高水位
参考答案：D
C级洁净室的换气次数要求是（）次/小时。
A.≥45
B.≥35
C.≥25
D.≥15
Not until he called the secretary three times ______ that the manager went to an important meeting. [ ]
A. did he tell
B. he told
C. was he told
D. he was told
