What Do Astronomers Mean When They Refer to "Gaps" in the Asteroid

Multiple Choice

What do astronomers mean when they refer to "gaps" in the asteroid belt between Mars and Jupiter?

A) There are very few asteroids with diameters between 1 and 100 kilometers, creating a "gap" in the size distribution.
B) Asteroids typically cluster together and this creates "gaps" on the sky.
C) Asteroids seem to avoid certain orbits around the Sun, creating "gaps" in the orbits that asteroids can have.
D) There is a large population of asteroids too faint to see called the "gap" asteroids.
E) There are either pure metal or pure rock asteroids, but no mixtures. Thus there is a "gap" in the composition of asteroids.