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 taps a and b can fill a cistern in 12 minutes and 18 minutes respectively. they are turned on at the same time. if the tap a is turned off after 4 minutes, how long will tap b take to fill the rest of the cistern ?
Options: a. 8 min., b. 9 min., c. 10 min., d. 7 min., e. none of these

Ex Output:
a


Ex Input:
Problem: for 2 consecutive yrs, my incomes are in the ratio of 4: 7 and expenses in the ratio of 3: 5. if my income in the 2 nd yr is rs. 42000 & my expenses in the first yr in rs. 23000, my total savings for the two - year is
Options: a. s. 8000, b. s. 9000, c. s. 9900, d. s. 9990, e. s. 10000

Ex Output:
a


Ex Input:
Problem: a garrison of 2000 men has provisions for 54 days. at the end of 18 days, a reinforcement arrives, and it is now found that the provisions will last only for 20 days more. what is the reinforcement ?
Options: a. 1977, b. 1600, c. 1979, d. 1900, e. 1278

Ex Output:

b