Why Will a Smaller Body (The Moon, for Example) Cool

Why will a smaller body (the Moon, for example) cool much faster than a larger body like Earth?

A) A smaller body has a larger surface area for heat to escape through, relative to its volume.
B) A smaller body will have a smaller proportion of radioactive elements.
C) A smaller body is less likely to have an atmosphere than a larger body.
D) A smaller body will have a smaller iron core, which holds heat better than rock.