A can do a piece of work in 4 days. B can do it in 7 days. With the assistance of C they completed the work in 2 days. Find in how many days can C alone do it?
Choose the correct option: A) 22 days B) 28 / 3 days C) 67 days D) 17 / 5 days E) 18 days

Rationale: "c = 1 / 2 - 1 / 4 - 1 / 7 = 3 / 28 = > 28 / 3 days answer : b"
Answer: B