A and B together can do a work in 9 days. If A alone can do it in 18 days. In how many days can B alone do it?	Choose the most appropriate option:	A) 10	B) 99	C) 18	D) 55	E) 21

Rationale: "1 / 9 – 1 / 18 = 1 / 18 = > 18 answer : c"
Answer: C