In the late 19th century, scientists began to classify bacteria based on phenotypic markers, such as the presence or absence of cilia. The distinction of prokaryotes and eukaryotes was introduced in the 1960s, but it wasn’t until 1977 that the Archaea were classified as a group separate from Bacteria. Today's scientists use phenotypes, genotypes, and phylogenic trees to determine relationships between organisms in Bacteria and Archaea.

Which of the following best explains why Bacteria and Archaea were not recognized as separate groups until 1977?

A. Scientists were able to access the extreme environments in which Archaea thrive and were able to obtain better samples to study.
B. More powerful microscopes were developed, which were needed to identify archaea.
C. Scientists became able to analyze DNA and derive better understandings of evolutionary relationships.
D. New species of prokaryotes evolved during these years.