Tara: This is Tara Patel from Cotton House in Kidderminster. Could I speak to Mr. Smith, please?

Mr. Smith: ______[ ]

A. Hello.

B. I'm Mr. Smith.

C. Who are you?

D. Speaking.

仪表稳压器的作用是在电源电压变化时稳定仪表（）电压，避免仪表的指示误差。