Definition: 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: Problem: a can do a half of certain work in 70 days and b one third of the same in 35 days. they together will do the whole work in ?
Options: a. 1 / 65, b. 1 / 62, c. 1 / 63, d. 1 / 60, e. 1 / 61
Output:
d