一道填空题：Mostpeopledon'tthinkofastampasareceipt,butthat'sitreallyis--aproofofjusthowmuchmoneyyouhavepaidinadvanceformaildelivery.空白处填what还是why?人大12年研究生考试题库上解释为why,我总

Question

一道填空题：Most people don't think of a stamp as a receipt,but that's___it really is--a proof of just how much money you have paid in advance for mail delivery.空白处填what 还是why?
人大12年研究生考试题库上解释为why,我总感觉应该是what,你们觉着呢?

Answer 1

还是应该填Why
it really is a proof of just how much money you have paid in advance for mail delivery
这是个完整的句子.
你用what的话,从句中必定缺少成分需要what去替代.