2・123 Two mailmen start from the post office at the same time, with A going up the mountain road to deliver mail and then returning the same way, and B going along the level road to the train station to deliver mail and then returning the same way. If their speeds on the level road are the same, the speed going up the mountain is 5 km/h slower than on the level road, and the speed going down the mountain is 5 km/h faster than on the level road, the distance from the post office to the mountain top and to the train station is 5 km each, and the time spent by the two mailmen delivering mail is negligible. Then
(A) A returns to the post office first.
(B) B returns to the post office first.
(C) A and B return to the post office at the same time.
(D) Any of the first three answers could be true.
(2nd "Five Sheep Cup" Junior High School Mathematics Competition, 1990)
[Solution] Let the speed on the flat road be $v$ km/h. According to the problem, the time required for Jia to make a round trip is $\frac{5}{v-5}+\frac{5}{v+5}=\frac{10 v}{v^{2}-25}=\frac{10}{v-\frac{25}{v}}>\frac{10}{v}$ (the time required by Yi), so Yi arrives at the post office first.
Therefore, the answer is $(B)$.