A virus is combined with a host cell in molten nutrient agar and then poured over a sterile nutrient agar plate. After the agar solidifies and the plate is incubated you see distinct clear zones in the agar. What do these results mean? A. The virus can replicate in nutrient agar. B. The virus can infect the host cells. C. The virus is unable to infect the host cells. D. The nutrient agar plate was contaminated in the process of setting it up.

The correct answer is B. The virus can infect the host.

Explanation:

As the host and the virus both remain in the agar plate, the virus is capable of infecting the host cells. The host cell multiplies and grows by utilising the nutrient from the agar medium. After the virus particles infect the host, they replicate inside the host and produce new progeny virions which get released out of the host cell by killing it. The newly formed virions infect other host cells and the process continues. The killing of the host cells by the viruses result in the generation of clear zones on the agar plate which is also known as the zone of exclusion. In the zone of exclusion region, the host cells have been killed by the viruses.