Why Are Parasites So Difficult to Classify Based on Morphology

Why are parasites so difficult to classify based on morphology only?

A) Parasites are so small that they are hard to study.
B) Convergent evolution often occurs due to the constraints of the parasitic lifestyle.
C) Fully parasitic animals are all members of the same phylum.
D) Parasitic animals are unicellular and have lost many of the typical animal features.