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 average weight of a group of persons increased from 48 kg to 51 kg, when two persons weighing 78 kg and 93 kg join the group. find the initial number of members in the group ?
Options: a. 87, b. 67, c. 23, d. 75, e. 25
Example Output: c

Example Input: Problem: a can do a job in 15 days and b in 20 days. if they work on it together for 4 days, then the fraction of the work that is left is ?
Options: a. 2 / 15, b. 8 / 15, c. 3 / 11, d. 1 / 12, e. 6 / 13
Example Output: b

Example Input: Problem: 10 % people of a village in sri lanka died by bombardment, 15 % of the remainder left the village on account of fear. if now the population is reduced to 2907, how much was it in the beginning ?
Options: a. a. 3800, b. b. 4200, c. c. 4400, d. d. 4500, e. e. 4600
Example Output:
a