在用李萨如图形法测音叉频率时，应将示波器调节为（）显示模式。

A.检查示波器面板显示旋钮是否在“double”档上 Check whether the display knob on the oscilloscope panel is in the "double" position

B.检查连接示波器线路是否接好 Check whether the circuit connecting the oscilloscope is well connected

C.适当改变信号发生器信号频率和振幅 Properly change the frequency and amplitude of signal generator signal

D.上下调节示波器显示位置按钮，观察屏幕波形是否移动 Adjust the oscilloscope display position button up and down to observe whether the screen waveform moves.

A.500Hz

B.2000Hz

C.4000Hz

D.1000Hz