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'.

Example Input: Problem: a swimmer can swim in still water at 4 km / h . if the speed of the water current is 2 km / h , how many hours will the swimmer take to swim against the current for 5 km ? 
Options: a ) 2.0 , b ) 2.5 , c ) 3.0 , d ) 3.5 , e ) 4.0
Example Output: b

Example Input: Problem: a man rows his boat 75 km downstream and 45 km upstream , taking 2 1 / 2 hours each time . find the speed of the stream ? 
Options: a ) 6 , b ) 7 , c ) 5 , d ) 8 , e ) 9
Example Output: a

Example Input: Problem: if 20 men can build a wall 66 metres long in 6 days , what length of a similar can be built by 86 men in 8 days ? 
Options: a ) 53 m , b ) 46 m , c ) 94 m , d ) 49 m , e ) 64 m
Example Output:
d