Remember toyour mobile phone when the plane takes off.　　A. turn on B. turn down C. turn of

问题补充：

Remember toyour mobile phone when the plane takes off.

A. turn on B. turn down C. turn off

答案：

解析： 词义辨析。turn on打开；turn down关小，调低；turn off关闭。句意：当飞机起飞时记得关掉手机。所以选C。