為啥arm前用an不用a

因為arm的首字母a是母音，例如：

1、He had had an arm amputated and relied on her to take down his books from dictation.他的一隻胳膊截掉了，只能口授書的內容讓她來記錄。

2、He has lost the use of an arm.他的一隻胳膊殘廢了。