Read and choose. 阅读短文内容， 选择正确的答案。

Mary is a little girl. She is only five years old. She is not at school. She doesn't know how to read or write. But her sister Joan is a schoolgirl. She is ten. She knows how to read and write.

One day, Joan sees her little sister in the room. She is at the table. There is a pencil in her hand. She is writing. "What are you writing, Mary?" She asks. "I'm writing to my friend, Rose." "But how can you? You don't know how to write." Says her sister. "Well," says Mary, "It doesn't matter. Rose doesn't know how to read, either."