Carbon dioxide is an excellent absorber and emitter or infra-red energy (IR). As a result, the presence of CO2 will allow an atmosphere to keep the temperature of a planet warmer than it would be in the absence of the atmosphere. This is known as the Greenhouse Effect. The radiative equilibrium temperature of a planet (what we called the "cue-ball temperature") is based SOLELY on the distance the planet is from the sun and so takes no account of the radiative properties of the atmosphere. Therefore, the radiative equilibrium temperature of Planet X is LOWER THAN its average temperature. The correct answer is C.