Why Do We Sometimes Observe Asteroids at the Distances of the Gaps

Multiple Choice

Why do we sometimes observe asteroids at the distances of the gaps in the asteroid belt?

A) They are kept in place by shepherding asteroids.
B) A gap is located at an average orbital distance, and asteroid orbits often have large eccentricities.
C) Jupiter's gravitational tugs keep them there.
D) They are held in place by resonances with other asteroids.
E) Actually, we never see asteroids in the gaps.