You will be given a definition of a task first, then some input of the task.
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.

Problem: a can do a piece of work in 15 days. a does the work for 5 days only and leaves the job. b does the remaining work in 6 days. in how many days b alone can do the work ?
Options: a. 5 days, b. 7 days, c. 12 days, d. 9 days, e. 10 days
Output:
d