In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.
Q: Problem: after working for 6 days , david was joined by moore . together they completed the remaining job in 3 days . how many days will it take both of them to complete the entire job , given that it would have taken david 12 days to complete the job alone ?
Options: a ) 7 , b ) 4 , c ) 6 , d ) 2 , e ) 7
A:
c