能量多普勒的优点是（）
A.帧频更高
B.对微弱的多普勒信号更敏感
C.计算流速更准确
D.计算流量更准确
E.混叠
参考答案：B
— How many books _____ they _____?
— Five. But they haven't finished reading even one. [ ]
A. did…borrow
B. had…borrowed
C. will…borrow
D. do…borrow
井下配电网路校难开关节设备的分断能力采用（）
A、最大三相短路电流
B、最小两相短路电流
C、长时负荷电流
