I knew a guy who played his radio only in the morning.

When someone asked him why，he replied, "This is an AM radio."

Notes:

1. I knew a guy who played his radio only in the morning.

我认识一个人，他只在早上收听广播。

2. When someone asked him why，he replied, "This is an AM radio."

当有人问他为什么时，他回答：“(我的)这一台是AM(调幅)收音机。

"AM"表示“调幅”(amplitude modulation)，也可表示“上午的”，通常与时间连用，不需再加o'clok,也可以用"a.m."表示，如：at 9 a.m.(在早上九点)。广播的收听方式有调幅 (AM)和调频(FM/frequency modulation)两种。

那个拥有AM收音机的人认为AM是早上的意思，所以只能在早上收听。

amplitude [ˈæmplɪtju:d] n.(物理)振幅

modulaiion [ˌmɒdjʊ'leɪʃn] n.(电子)电波频率的调变;调幅

frequency [ˈfri:kwənsi] n.(物理)频率