Thank you GG and MH. You all(or 'two'?) are well at searching for something by search engine, but MH's skill seems better(or seems to be better?). Kiding(or 'just have a joke/kid.'?). Maybe you often use the search tools, just like me. Actually, but, if you ask me that what is preceded a "vowel-starting word"(I am not sure about this describle included the quotation marks), I will answer(or follow by a 'to'?) you, "an" gets my vote. But I usually forget this rule when I write an article, maybe the pratice of writing is so poor to me(or 'maybe I lack the pratice of writing'?). So after(under?) your commented(comments?), I will take a closer attention to it (what is another expression), and thank you for you two teacher's comments very much again.
PS: Please fell free to check my grammar and tense, cause I am not sure about them.Thanks a lot again.