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: two trains of equal length are running on parallel lines in the same directions at 46 km / hr. and 36 km / hr. the faster trains pass the slower train in 72 seconds. the length of each train is :
Options: a. 82 m, b. 50 m, c. 72 m, d. 100 m, e. none of these
Example Output: d

Example Input: Problem: the present worth of rs. 2310 due years hence, the rate of interest being 15 % per annum, is :
Options: a. rs. 1440, b. rs. 1750, c. rs. 1680, d. rs. 1840, e. none of these
Example Output: c

Example Input: Problem: veena ranks 73 rd from the top in a class of 198. what is her rank from the bottom if 22 students have failed the examination ?
Options: a. 88, b. 104, c. 110, d. 90, e. 93
Example Output:
b