要调节音量，只需转动收音机上的黑色旋钮。

To adjust the volume, simply turn the black ___knob on the radio.

那栋老房子的前门上有一个漂亮的黄铜门把手。

The old house had a beautiful brass ___knob on its front door.

停电时，我无助地扭动着音响上的旋钮。

When the power went out, I was helplessly twisting the ___knob on the stereo.