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: a grocer has a sale of rs. 5420, rs. 5660, rs. 6200, rs. 6350 and rs. 6500 for 5 consecutive months. find the sale he should have in the sixth month, so that he gets an average sale of rs. 6400 ?
Options: a. rs. 5870, b. rs. 8270, c. rs. 6020, d. rs. 6850, e. none of these

Ex Output:
b

Ex Input:
Problem: find the value of 1 / ( 3 + 1 / ( 3 + 1 / ( 3 - 1 / 3 ) ) )
Options: a. 21 / 29, b. 27 / 89, c. 89 / 21, d. 29 / 21, e. none

Ex Output:
b

Ex Input:
Problem: a vessel of capacity 60 litres is fully filled with pure milk. nine litres of milk is removed from the vessel and replaced with water. nine litres of the solution thus formed is removed and replaced with water. find the quantity of pure milk in the final milk solution ?
Options: a. 23.89, b. 72.9, c. 38.3, d. 43.35, e. 79.3

Ex Output:
d