在病例对照试验中，选择对照的要求是（）。
A.未患某病的人
B.病例来源的人群中未患某病的，其他特征与病例组相同
C.病例来源的人群中未患所研究的疾病，某些可能影响患病的因素与病例组具有可比性的人
D.未患某病的，其特征与病例组相同的人
E.未患某病，与病例组具有同质的人
参考答案：C
解析：对照选择要求候选对象必须来自产生病例的总体，不患有所要研究疾病甚至是同一系统或同一病因的疾病。对照一旦发生所研究的疾病便成为病例组的研究对象。
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| One day, a poor boy who was trying to pay his way through school by selling goods door to door found that he only had one dime (一角银币) left. He was hungry so he decided to 1 for a meal at the next house. However, he lost his nerve when a lovely young woman opened the door. 2 a meal he asked for a drink of water. She thought he looked 3 so she brought him a large glass of milk. He drank it slowly, and then asked, "How much do I 4 you?" "You don't owe me anything," she replied. "Mother has taught me never to accept 5 for a kindness." He said, "Then I thank you from the bottom of my heart." As Howard Kelly left that house, he not only felt stronger 6 , but it also increased his faith in God and the human race. He was about to give up and 7 before this point. Years later the young woman became critically ill. The 8 doctors were baffled (阻挡). They finally sent her to the big city, where specialists could be called in to study her 9 disease. Dr Howard Kelly, now 10 was called in for the consultation (会诊). When he heard the name of the town she came from, a(n) 11 light filled his eyes. 12 , he rose and went down through the hospital hall into her room. Dressed in his doctor's gown he went in to see her. He 13 her at once. He went back to the 14 room and determined to do his best to save her life. From that day on, he gave special 15 to her case. After a long struggle, the battle was 16 . Dr Kelly requested the business office to pass the final bill to him for approval. He looked at it and then 17 something on the side. The bill was sent to her room. She was 18 to open it because she was sure that it would take the rest of her life to pay it off. Finally she looked, and the note on the side of the bill 19 her attention. She read these words: "Paid in full with a glass of 20 ." (Signed) Dr Howard Kelly Tears of joy flooded her eyes as she prayed silently:"Thank you, God. Your love has spread through human hearts and hands." | ||||
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