9.0 m$$^3$$ of sodium chloride solution is to be applied at a rate of 40 litre/m 2 /min for regeneration of an ion exchange unit. The approximate time required for simultaneous regeneration of five units, each having the diameter of 2.0 m and a height of 1.8 m. will be _______________

A

1.25 hours

B

2.5 hours

C

4.5 hours

D

6.25 hours