Machine A can finish a job in 4 hours, Machine В can finish the job in 12 hours, and Machine С can finish the job in 8 hours. How many hours will it take for A, B, and С together to finish the job? Choose the most appropriate option.
A) 15 / 4
B) 18 / 7
C) 21 / 8
D) 24 / 11
E) 28 / 13

Rationale: "the combined rate is 1 / 4 + 1 / 12 + 1 / 8 = 11 / 24 of the job per hour . the time to complete the job is 24 / 11 hours . the answer is d ."
Answer: D