prep51题 答案是D不明白 哎~

51.   6587-!-item-!-187;#058&005111

Circular gears P and Q start rotating at the same time at constant speeds.  Gear P makes 10 revolutions per minute, and gear Q makes 40 revolutions per minute.  How many seconds after the gears start rotating will gear Q have made exactly 6 more revolutions than gear P ?

(A) 6

(B) 8

(C) 10

(D) 12

(E) 15