Available choices:
(A). principals of aerodynamics;
(B). Physical weathering;
(C). floods;
(D). the tilt of the planet;
(E). an external framework;
(F). dry conditions;
(G). axis rotation;
(H). relative location;
Q: Fact 1: Earth orbiting the Sun causes seasons to change.  Fact 2: Seasons are caused by the tilt of the axis relative to the planet's orbit.  Given the two facts above, what causes earth's seasons?
A:
(D).