Viruses are unique among infectious agents because they are?

a. microscopicb. deadly

c. harmless

d. not living

It's answer d, but for info:

The question of wether or not a virus is alive is not resolved, some scientists say yes, some say no.

The fact is, viruses need another organism's enzymatic machinery to survive : alone, they cannot multiply or grow, produce or use energy, functions that are common to everything alive.

On the other hand, they do possess nucleic acids and proteins like living beings!