What is the overpaid portion of the rent?

During the year ended 31st December 2001,
Nuju Enterprises paid rent as follows:
Jan. 20, N24 000 for a period of 4 months.
May. 16, N30 000 for a period of 5 months.
Oct. 15, N36 000 for a period of 6 months.

A. N36 000
B. N18 000
C. N12 000
D. N6 000

Correct Answer:

Option B – N18 000