Convex quadrilateral ABCD has AB = 18, \angle{A} = 60 \textdegree, and AB∥CD. In some order, the lengths of the four sides form an arithmetic progression, and side AB is a side of maximum length. The length of another side is a. What is the sum of all possible values of a?<spanclass=′latex−bold′>(A)</span>24<spanclass=′latex−bold′>(B)</span>42<spanclass=′latex−bold′>(C)</span>60<spanclass=′latex−bold′>(D)</span>66<spanclass=′latex−bold′>(E)</span>84