阅读短文，下列问题

A

    It's two weeks before Christmas in England and Mrs. Smith is very busy. She buys a lot of Christmas cards to send to her friends and to her husband's friends. She puts them on the table in the room. Then, when her husband comes home from work, she says to him, “These are the Christmas cards for your friends. Could you please write the cards and I cook supper?”

    Mr. Smith doesn't say anything and walks into his study. After a while, he comes with a box full of Christmas cards. He puts them on the table and says, “They are from last year. I forgot(忘记) to send them.”

有一包铜和碳组成的固体混合物，在一定条件下与氧气完全反应，生成固体氧化铜和二氧化碳气体，测得反应前后固体质量不变，则原固体混合物中碳的质量分数是（　　）

如图，四边形ABCD是平行四边形，作AF∥CE，BE∥DF．

求证：BE=DF．