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: at what rate percent per annum will a sum of money double in 6 years.
Options: a. 12.5 %, b. 13.5 %, c. 11.5 %, d. 14.5 %, e. 16.6 %
Example Output: e

Example Input: Problem: a man goes downstream at 10 kmph, and upstream 8 kmph. the speed of the stream is
Options: a. 0 kmph, b. 4 kmph, c. 1 kmph, d. 2.5 kmph, e. 26 kmph
Example Output: c

Example Input: Problem: a circular ground whose diameter is 70 metres, has a 2.8 metre - broad garden around it. what is the area of the garden in square metres ?
Options: a. 540.72, b. 740.64, c. 640.64, d. 620.45, e. 725.26
Example Output:
c