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: a can do a job in 15 days and b in 20 days. if they work on it together for 4 days, then the fraction of the work that is left is ?
Options: a. 2 / 15, b. 8 / 15, c. 3 / 11, d. 1 / 12, e. 6 / 13
Example Output: b

Example Input: Problem: the circumference of the front wheel of a cart is 30 ft long and that of the back wheel is 32 ft long. what is the distance traveled by the cart, when the front wheel has done five more revolutions than the rear wheel ?
Options: a. 20 ft, b. 25 ft, c. 750 ft, d. 900 ft, e. 2400 ft
Example Output: e

Example Input: Problem: a tank contains 7,000 gallons of a solution that is 5 percent sodium chloride by volume. if 2,500 gallons of water evaporate from the tank, the remaining solution will be approximately what percent sodium chloride ?
Options: a. 4.77 %, b. 5.77 %, c. 6.77 %, d. 7.77 %, e. 8.77 %
Example Output:
d