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: in what time will a railway train 50 m long moving at the rate of 36 kmph pass a telegraph post on its way ?
Options: a. 5 sec, b. 4 sec, c. 3 sec, d. 6 sec, e. 1 sec
Example Output: a

Example Input: Problem: what is the sum of natural numbers between 30 and 100
Options: a. 4615, b. 4860, c. 5000, d. 5500, e. 5550
Example Output: a

Example Input: Problem: a courtyard is 25 meter long and 16 meter board is to be paved with bricks of dimensions 20 cm by 10 cm. the total number of bricks required is :
Options: a. 16000, b. 18000, c. 20000, d. 22000, e. none of these
Example Output:
c