Star Clusters of the Same Type and Structure Appear to Become

Star clusters of the same type and structure appear to become fainter than expected, on the basis of the inverse-square law alone, as distance from the Sun increases. This is because:

A) star clusters are systematically smaller and hence less bright, the farther they are from the galactic center and hence from the Sun.
B) photons of light become "tired" and appear less bright, the farther they travel.
C) some of the light is scattered and absorbed by interstellar dust and gas between distant clusters and Earth.
D) the cosmological redshift has moved some of the light into the infrared spectral region.