A circle has been divided into 6 sectors of 6 different sizes. Then 2 of the sectors are painted red, 2 painted green, and 2 painted blue so that no two neighboring sectors are painted the same color. One such coloring is shown below.
How many different colorings are possible?
Answer Choices:
A. 12
B. 16
C. 18
D. 24
E. 28