短文改错。

     A boy who was cleaning the shoes in the street said to a young man passed by, "Let me clean your boots.

It just spends you only a penny." But the man refused. Then the boy told him that he will clean his boots for

nothing. The man agreed, and soon one of boot shone brightly. Then he put the other boot on the box, and

the boy refused to clean it if he was paid two pence for his work. The young man refused pay for anything and

went away. But the well-cleaning boot made the dirty one so bad that he could not walk on. He returned back

and gave the boy two pence.

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计算麻疹疫苗接种后作血清检查的阳转率，分子为（）
计算麻疹疫苗接种后作血清检查的阳转率，分母为（）

A.麻疹易感人群

B.麻疹患者数

C.麻疹疫苗接种人数

D.麻疹疫苗接种后的阳转人数

E.麻疹疫苗接种后的阴性人数