他抓住黄铜门把手，打开了门。

He grasped the brass ___knob and opened the door.

转动黑色旋钮来打开收音机。

Turn the black ___knob to switch on the radio.

当我转动旋钮时，机器由于停电没有启动。

When I turned the ___knob, the machine didn't start because of a power outage.