A can do a piece of work in 40 days; B can do the same in 40 days. A started alone but left the work after 10 days, then B worked at it for 10 days. C finished the remaining work in 10 days. C alone can do the whole work in?
Choose the correct choice from the following choices:
A) 20 days  B) 65 days  C) 86 days  D) 45 days  E) 17 days

Rationale: "10 / 40 + 10 / 40 + 10 / x = 1 x = 20 days answer : a"
Answer: A