Task: Question Answering
Input (English): Question: A, B and C can do a piece of work in 20, 30 and 60 days respectively. In how many days can A do the work if he is assisted by B and C on every third day?

 Option A: 13 days

 Option B: 15 days

 Option C: 17 days

 Option D: 21 days

 Option E: None
Output (English): Option B