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: the perimeter of an isosceles right triangle is 12 + 12 sq rt 2. what is the length of the hypotenuse of the triangle ?
Options: ['a. 5', 'b. 6', 'c. 7', 'd. 8.485', 'e. 8']

Ex Output:
d


Ex Input:
Problem: the overall age of x and y is 19 year greater than the overall age of y and z. z is how many decades younger that x ?
Options: a. 11, b. 15, c. 12, d. 19, e. 20

Ex Output:
d


Ex Input:
Problem: two tains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. if the length of each train be 120 metres, in what time ( in seconds ) will they cross each other travelling in opposite direction ?
Options: a. 12, b. 14, c. 16, d. 20, e. 18

Ex Output:

a