An example of a plant virus is -

Mad cow disease in cattle is caused by an organism which has

Identify the part labeled as '$A$' in the given figure.

The capsid of tobacco mosaic virus $(TMV)$ has capsomers numbering:

There exists a close association between the algae and the fungus within a lichen. The fungus

$A$: Viruses are considered as a link between living and non-living organisms.
$R$: Viruses behave as inert,non-living particles in a free state,while they exhibit active,living behavior inside a host cell.