已知向量 a→ =（x，2）， b→ =（2，1）， c→ =（3，x），若 a→ ∥ b→ ，则 a→ • c&#

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已知向量 a→ =（x，2）， b→ =（2，1）， c→ =（3，x），若 a→ ∥ b→ ，则 a→ • c→ =（）

A：4

B：8

C：12

D：20

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D

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     John is a paper boy. He delivers（投递）newspapers to different houses in his street every day. He has about 80 customers（客户）. Half of his customers only take the newspapers on weekdays, and about half take the newspapers on weekdays and on Sundays. Two of John’s customers only take the newspapers on Sundays.
   John has to get up at 4：30 every morning to deliver his newspapers. It takes longer to deliver the newspapers on Sundays. The Sunday newspapers are twice as heavy as those on weekdays.
   John is saving his money to buy a new bicycle. He is also saving money for college （大学）. He has already saved 500 dollars

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