Given the task definition and input, reply with output. 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: arun and tarun can do a work in 10 days. after 4 days tarun went to his village. how many days are required to complete the remaining work by arun alone. arun can do the work alone in 30 days.
Options: a. 16 days., b. 17 days., c. 18 days., d. 19 days., e. 20 days.
c