An woman swims downstream 81 km and upstream 36 km taking 9 hours each time; what is the speed of the current? Choose the correct choice from the following:
A) 0.5 B) 1.5 C) 2.5 D) 3.5 E) 4.5

Rationale: 81 - - - 9 ds = 9 ? - - - - 1 36 - - - - 9 us = 4 ? - - - - 1 s = ? s = ( 9 - 4 ) / 2 = 2.5 answer : c
Answer: C