超声系统可控制的最大能量与最小能量之比为下列哪种
A．动态范围B．抑制C．增益D．放大因数E．分贝
参考答案：A
— I was worried about math, but Mr. Brown gave me an A!
— ________ That’s a difficult course.
A．Have a good luck.
B．Not really.
C．Congratulations!
D．You do?
— ______________________?
— They are in the woods.[ ]
A. Where are they
B. What are they doing
C. When are they
