Two window washers, ian and caleb , lean a ladder against the side of a building so that Ian can wash a window while Caleb holds the ladder. The top of the ladder reaches the window, which is 15 feet off the ground. the base of the ladder is 8 feet away from the building. How long is the ladder?
A. 18 feet
B. 16 feet
C. 17 feet
D. 15 feet