阅读短文。

    Billy and Sam were twins. They were in the same class. They were both very happy. One day their teacher asked the class to write a composition(作文)"My Mother". Sam wrote one, but Billy was lazy. He just copied his brother's. The next day, the teacher asked Billy why his composition was the same as Sam's. "We have the same mother, don't we?" answered Billy.

根据短文内容选择正确的答案。