What is the main difference between bacteria and viruses?

Bacteria and viruses are fundamentally different in their structure and classification. Bacteria are classified as prokaryotic cells, which means they are single-celled organisms that lack a defined nucleus and other membrane-bound organelles. They are complete living cells that can grow and reproduce independently in suitable environments. Viruses, on the other hand, are acellular entities, meaning they do not consist of cells and cannot carry out metabolic processes on their own. They require living host cells to replicate, hijacking the host's cellular machinery to produce new virus particles.

This distinction highlights why the answer is accurate: bacteria are categorized as living prokaryotic cells, while viruses exist outside the realm of cellular life and necessitate a host for propagation. The other options contain inaccuracies regarding cell structure and replication requirements. For example, bacteria can survive and reproduce independently, differing from viruses, which are reliant on host organisms for replication. Additionally, the classification of bacteria as eukaryotic or the presence of cell walls in bacteria and viruses is misleading and incorrect.