Detailed Instructions: 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'.
Q: Problem: a man rows his boat 110 km downstream and 90 km upstream , taking 3 hours each time . find the speed of the stream ? 
Options: a ) 6 , b ) 5 , c ) 7.5 , d ) 3.5 , e ) 2.5
A:
d