Which of the following gives the BEST statement of the Pythagorean theorem?

A. In a right triangle, the square of the length of the hypotenuse equals the sum of the square of the lengths of the legs.
B. In a right triangle, the square of the length of the hypotenuse equals the difference of the squares of the lengths of the legs.
C. If the square of the length c of one side of a triangle equals the sum of the square of the lengths of the legs, then it is a right triangle with the hypotenuse of length c.
D. If the square of the length c of one side of a triangle equals the difference of the square of the lengths of the legs, then it is a right triangle with hypotenuse of length c.