C is twice efficient as A. B takes thrice as many days as C. A takes 12 days to finish the work alone. If they work in pairs (i.e., AB, BC, CA) starting with AB on the first day, BC on the second day and AC on the third day and so on, then how many days are required to finish the work ?

[ A ]    6$1 \over 5$ days
[ B ]    4.5 days
[ C ]    5$1 \over 9$ days right
[ D ]    8 days
[ E ]    None of these
Answer : Option C
Explanation :
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