A and B can separately finish a piece of work in 20 days and 15 days respectively. They worked together for 6 days, after which B was replaced by C. If the work was finished in next 4 days, then the number of days in next 4 days, then the number of days in which C along could do the work is:

(a) 60 days

(b) 40 days

(c) 50 days

(d) 30 days