There are two pipes in a large water tank: Pipe C and Pipe D. Pipe C puts water into the tank while Pipe D spills water. If only Pipe C is opened, it will fill the tank in 2 hours. Meanwhile, if only Pipe D is opened, it will empty the tank in 3 hours. Suppose that for a particular purpose, both Pipe C and Pipe D are opened simultaneously; how many hours will it take for the tank to be full?