In the figure below, N congruent semicircles are drawn along a diameter of a large semicircle, with their diameters covering the diameter of the large semicircle with no overlap. Let A be the combined area of the small semicircles and B be the area of the region inside the large semicircle but outside the small semicircles. The ratio A: B is 1: 18. What is N ?
Answer Choices
A. 16
B. 17
C. 18
D. 19
E. 36