根据短文内容，从各题所给的A，B，C，D四个选项中选出最佳选项。
Christmas is coming. Stephen can see it everywhere he 1 There are Christmas things to sell in the shops and Christmas trees in the park. But it is only in late 2 . Stephen thinks this is wonderful. He likes Christmas very much. To celebrate it for a month is not enough.
He went into a clothes shop to buy 3 . He found a perfect (完美的) pair.
"Just these, sir?" asked the shop assistant.
"Yes, please," 4 Stephen.
"They look a little big for you. Are you sure they will 5 ?" asked the shop assistant.
"Oh, they will be fine," said Stephen,
It was 6 the trousers were a little big. But they 7 to be. Every Christmas Stephen 8 as Father Christmas and works in the local shopping mall. Children like to take photos with him.
Stephen walked out of the 9 and smiled happily. What 10 he do this Christmas?
的导函数为
，则数列
的前n项和为
