D. Gravity and Distance. The distance earth is from the sun is what's keeping us from barreling into the mass of flame and heat. The distance between us and the moon is what keeps the moon from crashing into us. It could not be C, as the definition of inertia is: "a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force." Hope this helps :)
