Every summer,the beautiful town welcomes top buskers,or stre

Every summer,the beautiful town welcomes top buskers,or street performers,____they're c alled ,from all over the world. A.as B.that C.wh ich D.what 就讲讲这题as和which的差别，简明些

as引导状语状语从句，修饰top buskers,or street performers整个句子，而which在这种情况只能做定语从句，而且只能修饰street performers
关于as和which的区别：
as 和which的相同点
(1) as 和which都可引导非限制性定语从句，放在主句之后；在非限制性定语从句中充当从句的主语、宾语或表语。如：
Tom has found a good job, as / which we all hope. 汤姆找到了一件好工作，这正是我们大家所希望的。
(2) as 和which都可代表整个句子的内容或主句中的某一成分。如：
His mother is ill, which makes him very sad. 他母亲病了，这件事情使他很难过。
He has made great progress, as / which makes us very happy. 他进步了，这使得我们很高兴。
以上两句中的as / which都指前面整个句子的内容。
Though he has lots of money, he still rides his old bike, which was bought ten years ago. 尽管他很有钱，但是他还骑着他的旧单车，这单车是十年前买的。
There you can meet many writers as you have ever read in books. 在那里，你可见到你在书中所读过的很多作家。

as与which的区别
(1) 当先行词前面有so + adj. + a / such + (a) + adj. , the same等词修饰时，引导词只能用as，而不用which。如：
She is so nice a girl as we all like. 她是个我们大家都喜欢的如此好的女孩。
It is such an interesting book as we all want to read. 它是本我们大家都想读的有趣的书。
上面两句中的as就不能用which替换。
(2) as 在从句中常作宾语；作主语时，从句的谓语动词常为系动词，即：系表结构或被动结构。如：
As you expected, he turned up on time. 正如你所愿，他按时到了。
As is said above, he has conquered the difficulty. 正如上面所说，他征服了困难。
(3) 当非限制性定语从句放到主句的前面时，引导词只能用as，而不用which。如：
As is planned, we got there before eight. 正如计划，我们再把店前到达了那儿。
(4) As引导的非限制性定语从句指表示积极的意义，与主句是顺理成章的关系，绝不能与之矛盾。若非限制性定语从句表示一种消极的意义，则只用which。如：
He has succeeded in his career, as / which we all hope. 他在事业中成功了，这正是我们大家所希望的。
Her grandma died last week, which made her very sad. 她的奶奶逝世了，这使得她很悲伤。
(5) 当非限制性定语从句的引导词前面有介词时，只能用which, 而不可用 as。如：
He has a new computer, for which he paid nearly ten thousand yuan. 他有一台新电脑，这台电脑花了他将近一万元。
From: http://www.hxen.com/grammar/dingyu/2011-07-15/149306.html
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