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: solution y is 30 percent liquid x and 70 percent water. if 2 kilograms of water evaporate from 6 kilograms of solution y and 2 kilograms of solution y are added to the remaining 4 kilograms of liquid, what percent of this new solution is liquid x ?
Options: a. 33 %, b. 35 1 / 2 %, c. 37 %, d. 40 %, e. 42 %
Example Output: d

Example 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
Example Output: a

Example Input: Problem: a diagonal of a polygon is an segment between two non - adjacent vertices of the polygon. how many diagonals does a regular 20 - sided polygon have ?
Options: ['a. 875', 'b. 170', 'c. 1425', 'd. 2025', 'e. 2500']
Example Output:
b