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: the area of a square field is 625 km 2. how long will it take for a horse to run around at the speed of 25 km / h ?
Options: a. 12 h, b. 10 h, c. 8 h, d. 4 h, e. none of these
Example Output: d

Example Input: Problem: the average of first 10 natural numbers is ?
Options: a. 5.6, b. 5.5, c. 5.1, d. 5.8, e. 5.2
Example Output: b

Example Input: Problem: the distance between west - town to east - town is 10 kilometers. two birds start flying simultaneously towards one another, the first leaving from west - town at a speed of 4 kilometers per minute and the second bird, leaving from east - town, at a speed of 1 kilometers per minute. what will be the distance, in kilometers, between the meeting point and west - town ?
Options: a. 3., b. 8., c. 10., d. 12., e. 15.
Example Output:
b