When written as P² = a³, Kepler's 3rd Law (with P in years and a in AU) is:
a) The square of the orbital period equals the cube of the semi-major axis.
b) The cube of the orbital period equals the square of the semi-major axis.
c) The square root of the orbital period equals the cube of the semi-major axis.
d) The cube root of the orbital period equals the square of the semi-major axis.