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: how many prime numbers are between 29 / 3 and 87 / 6 ?
Options: a. 1, b. 2, c. 3, d. 4, e. 5
Example Output: b

Example Input: Problem: a train of 25 carriages, each of 60 meters length, when an engine also of 60 meters length is running at a speed of 60 kmph. in what time will the train cross a bridge 1.0 km long ?
Options: a. 2.5, b. 3, c. 5, d. 7, e. 9
Example Output: a

Example Input: Problem: at the end of the day, february 14 th, a florist had 90 roses left in his shop, all of which were red, white or pink in color and either long or short - stemmed. a third of the roses were short - stemmed, 15 of which were white and 10 of which were pink. the percentage of pink roses that were short - stemmed equaled the percentage of red roses that were short - stemmed. if none of the long - stemmed roses were white, what percentage of the long - stemmed roses were red ?
Options: a. 20 %, b. 75 %, c. 50 %, d. 33.3 %, e. 80 %
Example Output:
d