This is an alarm clock 为什么要用an?

This is an alarm clock 为什么要用an? where is ( ) alarm clock?A an B a C the D this The alarm clock is an a____ invention. an alarm an.alarm. an alarm dresser,an,clock,the,there,is,on,alarm(.) There is —(a /an )alarm clock on the table . there is an alarm clock on the table.改为复数 This is a alarm clock 这个句子哪里错了?.. Is this an egg? this is an easy this is an Is this an eye? Is this an umbrella this is an Under the table is an alarm clock的同义句是什么? Where is the alarm clock?