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: two trains are moving in opposite directions at 60 km / hr and 90 km / hr. their lengths are 1.15 km and 0.6 km respectively. the time taken by the slower train to cross the faster train in seconds is ?
Options: a. 42, b. 9, c. 7, d. 67, e. 15
Example Output: a

Example Input: Problem: what is the units digit of 26 ! + 50 ! + 4 ! + 4 ! ?
Options: a. 8, b. 6, c. 4, d. 2, e. 0
Example Output: a

Example Input: Problem: the h. c. f. of two numbers is 20 and the other two factors of their l. c. m. are 13 and 14. the larger of the two numbers is :
Options: a. 276, b. 280, c. 322, d. 345, e. 354
Example Output:
b