单选题 Most people don't think of a stamp as a receipt, but that is ______ it really is a proof of just how much money you have paid in advance for mail delivery.
  • A. what
  • B. why
  • C. how
  • D. who
【正确答案】 B
【答案解析】[解析] 此题的难度在对句意的理解。