Earth and Mars Each Has About the Same Tilt of Its

Multiple Choice

Earth and Mars each has about the same tilt of its spin axis with respect to its orbital plane. Earth experiences precession, which causes the DIRECTION of its spin axis to change slowly over time, but the ANGLE of tilt stays relatively constant. By contrast, the ANGLE of tilt of Mars's spin axis is NOT expected to be stable over long periods of time. Which of these is a potential explanation for this?

A) Mars is a smaller planet, and smaller planets are all unstable in this manner.
B) Spin axes are stabilized by magnetism. Earth has a strong magnetic field and Mars has none.
C) Spin axes are stabilized by the presence of a large moon. Earth has one and Mars has none.
D) Mars was struck by a large object early in its history, accounting for the difference in its hemispheres. One aftereffect of this collision is a long-term instability in its spin direction.