—_______ do you talk to your plant?

—Well, I've heard that talking to plants is good for them. It can help them grow. [ ]

A. Why

B. Where

C. When

D. Who

医用超声波发生的原理是：

A．换能器的逆压电效应

B．换能器的正压电效应

C．换能器向人体发射电信号

D．换能器的热效应

E．换能器的辐射效应