an alarm clock中的冠词为什么要用an?而不能用a?

This is an alarm clock 为什么要用an? an alarm clock There is —(a /an )alarm clock on the table . The alarm clock is an a____ invention. take an interest in 为什么可以用an而不能用take interests in an American boy 为什么冠词用an? an.alarm. an alarm ----alarm clock and a CD _______on the desk A An are B A is “a unique opportunity”为什么冠词用a不用an? nba all star 前面的冠词能不能用an “useful”前加冠词时,为什么加“a”而不加“an”?元音字母开头的单词前是不是一定要加“an”?