题目

What is language for? Some people seem to think it's for practicing grammar rules and earning lists of words—the longer the list, the better. That's wrong．Language is for the exchange（交流）of ideas and information．It's meaningless knowing all about a language if you can't use it freely. Many students I have met know hundreds of grammar rules, but they can't speak correctly or fluently（流利地）．They are afraid of making mistakes．One shouldn't be afraid of making mistakes when speaking a foreign language. Native speakers make mistakes and break rules, too．Bernard 若α∈（π2，π），且3cos2α=sin（π4-α），则sin2α的值为（　　） A、118B、-118C、1718D、-1718