题目内容：
根据下面内容，回答题： One day, a poor boy who was selling goods from door to door to pay his way through school found he had only one thin dime left, and he was hungry. He decided he would ask for a meal at the next house. However, he lost his nerve when a lovely young woman opened the door. Instead of a meal he asked for a drink of water. She thought he looked hungry so she brought him a large glass of milk. He drank it slowly,and then asked, "How much do I owe you? You don't owe me anything," she replied. He said, "Then I thank you from my heart. "
Years later that young woman became critically ill. The local doctors sent her to the big city,where they called in specialists to study her rare disease. Dr. Howard Kelly was called in for the consultation. When he heard the name of the town she came from, he rose immediately and went to her room. Dressed in his doctor's gown he went in to see her. He recognized her at once. He went back to the consultation room determined to do his best to save her life. From that day he gave special attention to the case. After a long struggle, the battle was won.
Dr. Kelly requested the business office to pass the final bill to him for approval. He looked at it and then wrote something on the edge. The bill was sent to her room. She feared to open it, for she was sure it would take the rest of her life to pay for it all. Finally she looked, and something caught her attention on the side of the bill. She read these words: "Paid in full with one glass of milk. "
The boy sold goods from door to door because_________. A．he wanted to help support his family
B．he needed money for school
C．he had to pay back a student loan
D．he did not have enough pocket money
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