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.

Ex Input:
Problem: what is the rate percent when the simple interest on rs. 720 amount to rs. 180 in 4 years ?
Options: a. 6.25, b. 7.25, c. 8.25, d. 9.25, e. 10.25

Ex Output:
a

Ex 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

Ex Output:
b

Ex Input:
Problem: the population of a bacteria colony doubles every day. if it was started 9 days ago with 2 bacteria and each bacteria lives for 12 days, how large is the colony today ?
Options: a. 512, b. 768, c. 1024, d. 2048, e. 4096

Ex Output:
c