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.

[EX Q]: Problem: how many 4 - digit numbers that do not contain the digits 2, 3, 4, 5, 6, or 7 are there ?
Options: a. 168, b. 176, c. 184, d. 192, e. 200
[EX A]: d

[EX Q]: Problem: each of the three people individually can complete a certain job in 3, 4, and 6 hours, respectively. what is the lowest fraction of the job that can be done in 1 hour by 2 of the people working together at their respective rates ?
Options: a. 1 / 3, b. 9 / 20, c. 8 / 15, d. 5 / 12, e. 2 / 3
[EX A]: d

[EX Q]: Problem: elvin ' s monthly telephone bill is the sum of the charge for the calls he made during the month and a fixed monthly charge for internet service. elvin ' s total telephone bill for january was $ 46 and elvin ' s total telephone bill for february was 76 $. if elvin ' s charge for the calls he made in february was twice the charge for the calls he made in january, what is elvin ' s fixed monthly charge for internet service ?
Options: a. $ 5, b. $ 10, c. $ 14, d. $ 16, e. $ 28
[EX A]:
d