What is it?[ ]
A. They are apples.
B. It is an apple.
C. It is a apple.
答案：B
调查研究过程中应客服哪三种错误认识？
诊断最常用的超声频率是
A．20Hz以上
B．20MHz以上
C．20KHz-20MHz
D．2-10MHz
E．1kMHz以下
