A, B, and C working alone can finish constructing a house in 6, 9, and 12 hours respectively. A starts alone and works for 1 hour. He is then joined by B and they work together for 1 hour. Finally C joins them and they all work together to finish the work. If they start working at 9 am, at what time will they finish?
Choose the correct choice from the following options:
A) 11:38 am
B) 11:49 am
C) 12:11 pm
D) 12:32 pm
E) 12:43 pm
A completes 1/6 of the job each hour.   B completes 1/9 of the work each hour.   C completes 1/12 of the work each hour.   A completes 1/6 of the work in the first hour.   Working together, A and B complete another 5/18 for a total of 8/18.   Working together, A, B, and C can complete 13/36 of the work each hour.   To complete 20/36, the time required is (20/36) / (13/36) = 20/13 hours.   The work will be completed at 12:00 plus 7*60/13 minutes which is about 12:32 pm.   The answer is 12:32 pm.
<result>D</result>
A, B, and C working alone can finish constructing a house in 6, 9, and 12 hours respectively. A starts alone and works for 1 hour. He is then joined by B and they work together for 1 hour. Finally C joins them and they all work together to finish the work. If they start working at 9 am, at what time will they finish?