eraser前加a还是an?

eraser前加a还是an? black eraser前加an 还是a an eraser 还是 a eraser eraser前面是加a还是an 一块橡皮翻译是a eraser还是an eraser a eraser（橡皮）还是an eraser?为什么? 橡皮的单数是a eraser还是an eraser eraser前加a还是an?我觉得读起来好像是元音音素,但是教辅上写的是a, orange前加an还是加a It`s an eraser.为什么加an不加a MP4前加a还是an不定冠词 MP4前加an,a还是the exciting前加a还是an 单词 uncle前加a还是an Asian前加an还是a American前加a还是an? 11前加a还是an information前加a还是an