Which of these describes Kepler's third law?
A. The square of the orbital period is proportional to the cube of the orbit's semimajor axis.
B. The cube of the orbital period is proportional to the square of the orbit's semimajor axis.
C. The square of the orbital period is proportional to the square of the orbit's semimajor axis.
D. The cube of the orbital period is proportional to the cube of the orbit's semimajor axis.
