Given the task definition and input, reply with output. In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Problem: a boats man can row in still water at speed of 7 km / hr . it takes 6 hours more to travel the same distance in upstream than in downstream if the speed of the river is 3 km / hr . what is the distance between the two destinations ?
Options: a ) 40 , b ) 87 , c ) 126 , d ) 18 , e ) 91
a