ABC is an equilateral triangle. D is a point inside △ABC such that AD \equal{} 8, BD \equal{} 13, and \angle ADC \equal{} 120^\circ. What is the length of DC?<spanclass=′latex−bold′>(A)</span>12<spanclass=′latex−bold′>(B)</span>13<spanclass=′latex−bold′>(C)</span>14<spanclass=′latex−bold′>(D)</span>15<spanclass=′latex−bold′>(E)</span>16