Structural Design Specifications - Engineering

Q1:

The minimum horizontal distance between two parallel main reinforcing bars, is

A the diameter of the bar if their diameters are same

B the diameter of the longer bar if their diameters are unequal

C 5 mm more than nominal maximum size of the coarse aggragate

D greatest value of the above.

ANS:D - greatest value of the above.

Please explain the answer.