闸板防喷器关井后，（）应进行锁紧。
A、需长期关井时
B、正常关井后
C、控制管路漏油时
参考答案：A
There ______ a lot of red-crowned cranes in other parts of the world many years ago. [ ]
A. is
B. are
C. was
D. were
下列哪种单板应用时受到色散影响最严重？（）
A.SOTU2.5G
B.SRM42
C.SOTU10G
D.GEM2
