The Figure Below Illustrates the Three-Domain, Six-Kingdom Taxonomy of Life

The figure below illustrates the three-domain, six-kingdom taxonomy of life.
Bacteria are equivalent at the kingdom and domain taxon; what explains this equivalency when the domain Eukarya is subdivided into four kingdoms?

A) Bacteria are small and relatively insignificant; most systematists focus on more important biological questions.
B) Bacteria are difficult to study, as more becomes known about them systematists will undoubtedly propose reclassifications.
C) All living bacteria are so similar that only a handful of families have been proposed.
D) At present, no significant shared derived features have been identified that would indicate major evolutionary divisions have occurred within the group.