4个相同的声压级迭加，总声级较单个增加（）分贝。
A.3
B.5
C.6
D.10
参考答案：C
--- ___________ there any sharks in the aquarium? --- No, we didn’t see any.
A．Did
B．Had
C．Have
D．Were
汽机轴向位移如何定位？
