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问题 单项选择题

丙烯制冷压缩机开车时，应在（）时将氮气密封改为自身密封。

A．透平暖机前

B．丙烯制冷压缩机出口压力大于氮气压力

C．调速器动作时

D．开车正常后

答案

参考答案：B

选择题
— The housework is too much for me, Mum. 
— Sorry, but I can't help ____ it, dear. I've got something important to do, you know.

A. having done
B. doing
C. but do
D. do
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填空题
若|
a
|=|
b
|=1，
a
⊥
b
，且2
a
+3
b
与k
a
-4
b
也互相垂直，则k的值为 ______．
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