A rectangular reservoir is filled with water till one-fifth of the height of the reservoir. If an outlet at the bottom of the reservoir is unplugged, the water in the reservoir will drain completely in 1 hour. If the outlet remains plugged and the inlet tap at the head of the reservoir is opened, the reservoir will fill to the brim in 2 hours. In how much time will the reservoir fill to the brim if the outlet is unplugged 30 minutes after the inlet tap is opened ?

A) 3 hours

B) 3 hours 30 minutes

C) 4 hours 30 minutes

D) 5 hours

E) The reservoir will never be filled to the brim

Can someone please explain the solution and approach ?