Working alone at its constant rate, machine K took 3 hours to produce $$\frac{1}{4}$$ of the units produced last Friday. Then machine M started working and the two machines, working simultaneously at their respective constant rates, took 6 hours to produce the rest of the units produced last Friday. How many hours would it have taken machine M, working alone at its constant rate, to produce all of the units produced last Friday?

A:8

B:12

C:16

D:24

E:30