At one time, sponges were lumped into one phylum. Then, they were separated into several different phyla. Now, they are considered one phylum again. These changes indicate which of the following?

A. The goal of making all taxonomic groups monophyletic is unrealistic.
B. We need more fossil evidence of sponges.
C. Molecular and morphological data often conflict when we try to reconstruct evolutionary history.
D. Every phylogeny should be considered a hypothesis that must be revised in the light of new data.