根据短文内容，选择正确答案。

C

    Betty and I are best friends. Our birthdays are on the same day, so every year we have a birthday party together. But this year we had a costume party instead.

    While we were writing the invitations, my mom came in and asked, "Nancy, what about inviting John?" John had been in our class for only a few months, but he was already getting better grades in math than anyone else in our class. "Mom, he wears the same pants to school every day. How could he even afford a costume?" Mom said nothing.

    The next day, mom gave me an envelope(信封) with a shopping card in it. "I thought it would be nice for you to give this to John," Mom said. But how? We didn't want to make John embarrassed. We discussed it for a long time. Finally, Betty and I had a good idea.

    On the day of our party, John arrived, in an old sheet(床单), but still in the same brown pants as usual. Before eating the birthday cake, Betty said in a loud voice, "Now it's time for the great prize game." It was a math game. None of us were surprised when John gave the right answer first and walked off with the envelope.

    Everything went well as we planned. John wore a new pair of pants and a new shirt the next week. He felt happy. So did we.