Task: Question Answering
Input (English): Question: Three pipes, A, B, & C are attached to a tank. A & B can fill it in 20 & 30 minutes respectively while C can empty it in 20 minutes. If A, B & C are kept open successively for 1 minute each, how soon will the tank be filled?

 Option A: 2 hours

 Option B: 4 hours

 Option C: 1 hour 30 min

 Option D: 5 hours

 Option E: 6 hours
Output (English): Option C