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: how many positive even integers less than 100 contain digits 5 or 7 ?
Options: a. 10, b. 17, c. 18, d. 19, e. 20

Ex Output:
a

Ex Input:
Problem: if the simple interest on a certain amount in at 4 % rate 5 years amounted to rs. 2240 less than the principal. what was the principal ?
Options: a. 1500, b. 2500, c. 2507, d. 2800, e. 11500

Ex Output:
d

Ex Input:
Problem: a person travels equal distances with speeds of 3 km / hr, 4 km / hr and 5 km / hr and takes a total time of 54 minutes. the total distance is ?
Options: a. 1 km, b. 2 km, c. 3 km, d. 3.4 km, e. 5 km

Ex Output:
d