A and B together can do a piece of work in 6 days and A alone can do it in 9 days. In how many days can B alone can do it?  Choose the correct choice from the following choices:
A) 12 days
B) 15 days
C) 18 days
D) 21 days
E) 22 days

Rationale: "explanation : a and b can do work 1 / 6 in 1 day a alone can do 1 / 9 work in 1 day b alone can do ( 1 / 6 - 1 / 9 ) = 1 / 18 work in 1 day = > complete work can be done in 18 days by b answer : option c"
Answer: C