Which of the following situations is a direct proportion? 

A. Fifteen men can do a job in 12 days while 5 men can finish in 36 days.

B. Three buses can carry 120 students while 9 buses can carry 360 students.

C. A wall can be built in 10 days by 12 workers. The same wall can be built in 24 days by 5 workers.

D. A family of four consumes a rice of sack in 36 days while 8 members of a family consumes in 18 days. ​