Pipe A can fill an empty tank in 30 hours while pipe B can fill it in 45 hours. Pipe A and B are opened and closed alternatively, i.e., first pipe A is opened, then B, again A and then B and so on for 1 hour each time without any time lapse. In how many hours will the tank be filled when it was empty, initially?

• A. 36 hr
• B. 54 hr
• C. 48 hr
• D. 60 hr