Which of the following states the Pythagorean theorem?
A. In a right triangle, the square of the hypotenuse equals the sum of the legs.
B. In a right triangle, the hypotenuse equals the sum of the squares of the legs.
C. In a right triangle, the hypotenuse equals the sum of the legs.
D. In a right triangle, the square of the hypotenuse equals the sum of the squares of the legs.