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: a, b and c, each working alone can complete a job in 6, 8 and 12 days respectively. if all three of them work together to complete a job and earn $ 1170, what will be b ' s share of the earnings ?
Options: a. $ 390, b. $ 520, c. $ 1080, d. $ 1170, e. $ 630
A:
a