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

Remember to your mobile phone when the plane takes off.　　
A. turn on B. turn down C. turn off

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。

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