uniform前为什么要加a

uniform前+a还是an The policemen are in uniform为什么uniform不加s uniform 前加的是a还是an ,哪umbrella 呢 wear a uniform为什么不用an "uniform"的前面是加"a",还是加"an"? we wear school uniform at school.uniform为什么不加s? a uniform还是an uniform "a uniform" or "an uniform"? I am worker 为什么要在worker前加a? world前为什么加冠词a I have uniform in my roomd在uniform前填什么冠词A an B a c some Do you wear a uniform at work?为什么用at