18. Which of the following sentence best describes why viruses are considered necessary intracellular pathogens?
(A) They can reproduce independently, but they tend to replicate faster inside cells.
(B) They lack ribosomes and mitochondria and must hijack host machinery to make new virions.
(C) They reproduce only through cell division and require host nutrients for energy.
(D) They are always larger than bacteria, so they must use host cells for transport.
(E) They can replicate their genome outside cells but require cells only for entry.