The 3-Domain, 6-Kingdom classification system has only been used for a few decades. The original scheme did not include the classification level of “Domain”, but this level was added after scientists discovered new unicellular prokaryotes. Why were these new organisms placed in the Domain Archaea, instead of being included in the Domain Bacteria? Group of answer choices

They can’t live in extreme environments

They were not single-celled.

They had a membrane-bound nucleus

They were older and structurally different than other prokaryotes.