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