根据短文理解, 从A、B、C、D四个选项中选择最佳答案填空。
Mabel is a beautiful girl. She likes all kinds of clothes, but she didn't1 her heart into her lessons when she was at school. With her friends' help, she finally finished middle school, 2 she learned little there. Of course, she can't do any important work and now she is a salesgirl in a 3 She has to work there and hopes to find a better one .
One afternoon the girl argued with her mother angrily. So the following morning, . When she served an old man, she was very4 The old man chose a pair of shoes and bought them. Before he left, he asked, "How 5 will the shoes last?"
"It depends. If you don't use them .the shoes will 6 be broken. "
The old man 7 nothing and went away. That afternoon, while Mabel was 8 for supper, she cut her finger. She had to go to a hospital. That old man happened to be on duty. After Mabel got the medicine from him, she asked, "When will I get 9 ?"
"It depends, "said the old man. "If you don't use 10 , you'll lose your finger. "
的定义域为（ ）