Anna is making a square pen for her rabbit to play in. Each
 side of the pen is 9 feet long.
 The formula for the area of a square is A = 52, where A
 stands for the area and s is the length of one side.
 What is the area of the pen?
 A. 9 square feet
 B. 36 square feet
 C. 18 square feet
 D. 81 square feet