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.

Example Input: Problem: 10 ^ ( 600 ) ã · 10 ^ ( 580 ) = ?
Options: a. 10 ^ 10, b. 10 ^ 20, c. 10 ^ 22, d. 10 ^ 23, e. 10 ^ 24
Example Output: b

Example Input: Problem: if john makes a contribution to a charity fund at school, the average contribution size will increase by 50 % reaching $ 75 per person. if there were 1 other contributions made before john ' s, what is the size of his donation ?
Options: a. $ 100, b. $ 150, c. $ 200, d. $ 250, e. $ 450
Example Output: a

Example Input: Problem: a man whose speed is 4.5 kmph in still water rows to a certain upstream point and back to the starting point in a river which flows at 1.5 kmph, find his average speed for the total journey ?
Options: a. 4, b. 5, c. 6, d. 7, e. 8
Example Output:
a