A2 km/hB3 km/hC1 km/hD4 km/h Step-by-Step Solution2/(10−s)+2/(10+s)=5/12 ⇒ 40/(100−s2)=5/12 ⇒ 100−s2=96 ⇒ s=2 km/h.Answer: A) 2 km/h