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 student travels from his house to school at 10 km / hr and reaches school 2 hours late. the next day he travels 18 km / hr and reaches school 1 hour early. what is the distance between his house and the school ?
Options: a. 65.0, b. 67.5, c. 70.0, d. 72.5, e. 75.0
Example Output: b

Example Input: Problem: 1391 x 1391 = ?
Options: a. a. 1951609, b. b. 1951601, c. c. 1951602, d. d. 1951603, e. e. 1934881
Example Output: e

Example Input: Problem: the area of circle o is added to its diameter. if the circumference of circle o is then subtracted from this total, the result is 4. what is the area of circle o ?
Options: ['a. 2 / π', 'b. 2', 'c. 8', 'd. 4', 'e. 4 π']
Example Output:
e