Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

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
d