The antibiotics have no effect on viruses because Viruses show metabolism of their own; Viruses are too small in size for antibiotics to act upon them; Viruses produce a thick covering and encyst themselves as endospore; Viruses show no metabolism of their own.

Antibiotics work by interfering with the metabolism generally, but viruses are parasites that can only reproduce inside a host cell. They have no metabolism of their own. Viruses use the molecular structures and functions of the infected cells. These are totally resistant to commonly used antibiotics. That's why antibiotics have no effect on viruses. So, the correct answer is option C.