每次放水量的大小，取决于放水器的（）。
A、容积
B、容积、抽放负压
C、抽放负压
参考答案：B
If the weather had been better, we could have had a picnic. But it _____ all day. [ ]
A. rained
B. rains
C. has rained
D. is raining
颈内动脉虹吸段一般分为（）
A.2段
B.3段
C.4段
D.5段
E.6段
