Task: Question Answering
Input (English): Problem: pipe p can fill a tank in 3 hours, pipe q in 9 hours and pipe r in 18 hours. if all the pipes are open, in how many hours will the tank be filled ?
Options: a. 2 hours, b. 2.4 hours, c. 3 hours, d. 3.5 hours, e. 4.5 hours
Output (English): a