Emir is standing in a treehouse and looking down at a swingset in the yard next door. The angle of depression from Emir's eyeline to the swing set is 31.43°, and Emir is 11 feet from the ground. How many feet is the base of the tree from the swingset? Round your answer to the nearest foot.

a. 15 feet
b. 20 feet
c. 18 feet
d. 24 feet