The variables of Kepler's 3rd law, 'a' and 'P' stand for: Group of answer choices 'a' is for amplitude in km; 'P' is for position in AU. 'a' is for amplitude in cm; 'P' is for pressure in N/m2. 'a' is for semi-major axis in AU; 'P' is for period in years. 'a' is for acceleration in m/s2; 'P' is for periodicity in seconds