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.

Let me give you an example: Problem: a multiple choice test consists of 4 questions, and each question has 5 answer choices. in how many r ways can the test be completed if every question is unanswered ?
Options: a. 24, b. 120, c. 625, d. 720, e. 1024
The answer to this example can be: c
Here is why: 5 choices for each of the 4 questions, thus total r of 5 * 5 * 5 * 5 = 5 ^ 4 = 625 ways to answer all of them. Hence, the answer is c.

OK. solve this:
Problem: working alone, a can complete a certain kind of job in 3 hours. a and d, working together at their respective rates, can complete one of these jobs in 2 hours. in how many hours can d, working alone, complete one of these jobs ?
Options: a. 2, b. 4, c. 6, d. 8, e. 10
Answer:
c