Enzymes are absent in

Statement $X$: Lichens are composed of two main components. Statement $Y$: In lichens,the phycobiont is the heterotrophic component and the mycobiont is the autotrophic component.

Which of the following bacteriophages is virulent?

$A$: Lichens do not grow in polluted areas having $SO_2$.
$R$: Lichens secrete carbonic acid and oxalic acid on barren rocks.

What are viruses parasitising bacteria called? Draw a well-labelled diagram of the same.

Mad cow disease in cattle and $Creutzfeldt-Jakob$ disease in humans are due to infection by