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.
Q: Problem: twelve machines, each working at the same constant rate, together can complete a certain job in 6 days. how many additional machines, each working at the same constant rate, will be needed to complete the job in 3 days ?
Options: a. 14, b. 12, c. 15, d. 16, e. 18
A:
b