Dan can do a job alone in 12 hours. Annie, working alone, can do the same job in just 9 hours. If Dan works alone for 4 hours and then stops, how many hours will it take Annie, working alone, to complete the job?  Select.
A) 2	B) 4	C) 6	D) 8	E) 10

Rationale: "dan can complete 1 / 12 of the job per hour . in 4 hours , dan completes 4 ( 1 / 12 ) = 1 / 3 of the job . annie can complete 1 / 9 of the job per hour . to complete the job , annie will take 2 / 3 / 1 / 9 = 6 hours . the answer is c ."
Answer: C