声波发射脉冲应为阶跃或矩形脉冲，电压幅值应为200～（）V。
A.500
B.800
C.1000
D.1500
参考答案：C
The store sells sweaters _____ a very good price. We can buy one ______ 40 yuan.
A．in; at
B．on; for
C．at; on
D．at; for
种子园
