The three side lengths of a triangle are in arithmetic progression with shortest side of length 6 One of the interior angles measures 120^{\circ}. What is the area of the triangle?

**Answer Choices**

A. 12 \sqrt{3}

B. 8 \sqrt{6}

C. 14 \sqrt{2}

D. 20 \sqrt{2}

E. 15 \sqrt{3}