Task: Question Answering
Input (English): Question: Two taps can separately fill a cistern 5 minutes and 7.5 minutes respectively and when the waste pipe is open, they can together fill it in 18 minutes. The waste pipe can empty the full cistern in?

 Option A: 3.8

 Option B: 3.7

 Option C: 3.6

 Option D: 3.9

 Option E: 3.5
Output (English): Option C