A. 12 days
B. 3 days
C. 7 days
D. 9 days
3 persons can do 3 jobs in 3 days
1/3 of the work will take 1/3 of the time, so
3 people can do 1 job in 1 day
It will take 1 person 3 times as long, so
1 person can do 1 job in 3 days
7 persons can do 7 times the work in the same time, so
7 persons can do 7 jobs in 3 days