What...for?的答语用 "Because..."还是 "For..."

What ...for = Why
故用because 好
如果不明白,请再问；如果对你有所帮助,
请点击本页面中的“选为满意回答”按钮,不能用for吗？不能用for吗？根据具体的语境也可以　　那么什么时候用because，什么时候用for呢？What did you buy this book for ?I buy it for reading .Because I want to read it .