阅读短文，根据短文内容选择正确的答案。
An old woman can't read or write. One day she asks a young man to write a letter for her. She wants to send the letter to her son. Her son lives in a city, not very far from here.
"I can't write your letter." says the man, "Because I have a bad leg."
"I know." says the old woman, "but you don't write with your legs." "Yes. But I can't walk so far and read the letter for your son."
"What do you mean, sir?" says the old woman. "My son can read it by himself."
"Don't you know?" answers the man, "that only I can read what I write."
