In quadrilateral A B C D, sides \overline{A B} and \overline{B C} both have length 10 , sides \overline{C D} and \overline{D A} both have length 17 , and the measure of angle A D C is 60^{\circ}. What is the length of diagonal \overline{A C} ?
Answer Choices
A. 13.5
B. 14
C. 15.5
D. 17
E. 18.5