单项选择题

A．It has broken down before.
B．She generally doesn’t allow people to borrow it.
C．She is trying to sell it.
D．She bought it from a friend.
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The Sabin vaccine was given most often on ______, but Koprowski’s vaccine was squirted into people’s mouths.
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Sabin and Koprowski created their vaccines by culturing a strain of polio virus on monkey kidneys, supposedly the source of ______, now called SIV.
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