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: 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

Ex Output:
a


Ex Input:
Problem: a goods train runs at the speed of 72 kmph and crosses a 250 m long platform in 24 seconds. what is the length of the goods train ?
Options: a. 230 m, b. 240 m, c. 260 m, d. 270 m, e. none of these

Ex Output:
a


Ex Input:
Problem: from a container having pure milk, 20 % is replaced by water and the process is repeated thrice. at the end of the third operation, the milk is ?
Options: a. 45 %, b. 49.6 %, c. 51.2 %, d. 55 %, e. 60.3 %

Ex Output:

c