Distinguish between bacteria and viruses.

• A. Bacteria are living, single-celled organisms; viruses are nonliving particles that require a host to replicate.
• B. Bacteria require a host to replicate; viruses replicate independently.
• C. Both bacteria and viruses are nonliving.
• D. Viruses are multicellular organisms.

Answer: (A)

The key idea is whether an organism can carry out life processes on its own. Bacteria are living, single-celled organisms with their own cellular machinery, metabolism, and the ability to reproduce independently (for example, by binary fission). Viruses, on the other hand, are nonliving outside a host cell; they lack cellular structure and metabolic activity and cannot replicate without hijacking a host’s cellular machinery. Thus, the description that bacteria are living and single-celled while viruses are nonliving particles that require a host to replicate best captures this fundamental difference. The other statements don’t fit: bacteria do not need a host to replicate, viruses do replicate only with a host, both are not nonliving, and viruses are not multicellular.