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: if julie rows 32 km upstream and 72 km down steam taking 4 hours each, then the speed of the stream
Options: a. 1.5 kmph, b. 0.5 kmph, c. 0.4 kmph, d. 1.6 kmph, e. 2.5 kmph

Ex Output:
b

Ex Input:
Problem: what sum of money will produce rs. 80 as simple interest in 4 years at 3 1 / 2 percent ?
Options: a. 263, b. 571.4, c. 367, d. 368, e. 339

Ex Output:
b

Ex Input:
Problem: a mixture of 70 litres of milk and water contains 10 % water. how many litres of water should be added to the mixture so that the mixture contains 12 1 / 2 % water ?
Options: a. 2, b. 8, c. 6, d. 3, e. 4

Ex Output:
a