A circle with center O has area 156 \pi. Triangle A B C is equilateral, \overline{B C} is a chord on the circle, O A=4 \sqrt{3}, and point O is outside \triangle A B C. What is the side length of \triangle A B C ?

**Answer Choices**

A. 2 \sqrt{3}

B. 6

C. 4 \sqrt{3}

D. 12

E. 18