Viruses a. usually infect only specific types of cells. b. reproduce on their own and sometimes within hosts. c. are nonliving particles. d. are able to crystallize.

The answer is letter B.

Viruses have the ability to reproduce on their own and sometimes within hosts. It is an infectious agent that can affect all types of life forms (e.g. animals, plants, microorganisms). They are found in almost every ecosystem on Earth and they are the most abundant type of biological identity.  Asvirology is the study of viruses which is a sub-specialty of microbiology. The spread in many ways and can be transmitted from one organism to another or sometimes to multiple organisms at the same time. Its origins in the evolutionary history of life is still unclear but they are considered to be life forms because of their capacity to carry genetic material, reproduce, and evolve through natural selection.