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【单选题】
甲、在一条椭圆形田径跑道上练习快跑和慢跑，甲的速度为3米／秒，乙的速度是7米／秒。甲、同一点同向跑步，经100秒第一次相遇，若甲、相反方向跑，经（ ）秒第一次相遇。A.
30
B.
40
C.
50
D.
70
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