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: a train 50 m long takes 6 sec to cross a man walking at 5 kmph in a direction opposite to that of the train. find the speed of the train ?
Options: a. 15 kmph, b. 25 kmph, c. 35 kmph, d. 45 kmph, e. 55 kmph
Example Output: b

Example Input: Problem: in what time will a train 120 m long cross an electric pole, it its speed be 121 km / hr ?
Options: a. 2.5, b. 2.7, c. 2.9, d. 3.6, e. 2.1
Example Output: d

Example Input: Problem: a metered rikshaw charges rs 13.50 for the first 1 km and 2.50 per 1 / 3 km there after. what would the charge if this rikshaw is hired for 13 kms ? in approx ? ?
Options: a. 103.5 rs, b. 104.5 rs, c. 105.5 rs, d. 106.5 rs, e. 107.5 rs
Example Output:
a