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.

Input: Consider 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

Output: a


Input: Consider Input: Problem: the total marks obtained by a student in mathematics and physics is 50 and his score in chemistry is 20 marks more than that in physics. find the average marks scored in mathamatics and chemistry together.
Options: a. 15, b. 25, c. 35, d. 45, e. 55

Output: c


Input: Consider Input: Problem: a bottle contains a certain solution. in the bottled solution, the ratio of water to soap is 3: 2, and the ratio of soap to salt is 4 times this ratio. the solution is poured into an open container, and after some time, the ratio of water to soap in the open container is quartered by water evaporation. at that time, what is the ratio of water to salt in the solution ?
Options: a. 12: 8, b. 8: 16, c. 9: 16, d. 8: 12, e. 16: 9


Output: c