The parallelogram bounded by the lines y=a x+c, y=a x+d, y=b x+c, and y=b x+d has area 18. The parallelogram bounded by the lines y=a x+c, y=a x-d, y=b x+c, and y=b x-d has area 72. Given that a, b, c, and d are positive integers, what is the smallest possible value of a+b+c+d ?
Answer Choices
A. 13
B. 14
C. 15
D. 16
E. 17