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: the area of a triangle is with base 18 m and height 6 m ?
Options: a. 88 m 2, b. 10 m 2, c. 66 m 2, d. 54 m 2, e. 31 m 2
Example Output: d

Example Input: Problem: if a certain coin is flipped, the probability that the coin will land heads is 1 / 2. if the coin is flipped 6 times, what is the probability that it will land heads up on the first 4 flips but not on the last 2 flips ?
Options: a. 1 / 8, b. 1 / 16, c. 1 / 32, d. 1 / 64, e. 1 / 128
Example Output: d

Example Input: Problem: in what ratio must tea at rs. 60 per kg be mixed with tea at rs. 70 per kg so that the mixture must be worth rs. 67.5 per kg ?
Options: a. 3: 1, b. 3: 2, c. 4: 3, d. 1: 3, e. none
Example Output:
d