B

    There was a bookseller(书商). He did not like to pay for anything. One day a big box of books fell on his foot. "Go to the doctor," said his wife, "and show that foot to him." "No," he said, "I'll wait until the doctor comes to our shop next time. Then I'll ask him about my foot. If I go to see him, I shall have to pay."

    The next day, the doctor came to the shop to buy some books. The bookseller told the doctor about his bad foot. The doctor looked at it and promised(答应)to help. He took out a piece of paper and wrote something on it. "Buy this medicine and put it on the foot before you go to bed every night," he said.

    "Thank you," said the bookseller, "And now, sir, here are your books."

    "How much?" asked the doctor.

    "Two dollars." "Oh, good," said the doctor, "I shall not have to pay you anything today."

    "Why?" asked the bookseller.

    "I looked over your foot. I want two dollars for that. If people come to my house, I ask them to pay only one dollar for a small thing like that. But when I go to their houses, I usually charge(收费)two dollars. And I came here today, didn't I? Bye bye."

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进行社会调查，首先要有明确的（    ）