$AIDS$ is a

In the given flow chart,the replication of a retrovirus in a host cell is shown. Observe it carefully and fill in the blanks $A, B$,and $C$.

How is $AIDS$ transmitted?

What is the correct sequence of events that occur after $HIV$ infection in a human body?
$(I)$ Viral $DNA$ enters the host $DNA$
$(II)$ Entry into $T_H$ lymphocytes and replication
$(III)$ Viral $RNA$ is converted into viral $DNA$ by reverse transcriptase
$(IV)$ $HIV$ enters into macrophages
$(V)$ Destruction of $T_H$ lymphocytes
$(VI)$ New virus particles are produced from the host cell

In which condition does the $CD_4$ count in the blood fall below $200 \times 10^{6}$ cells/$L$?

$AIDS$ is characterized by a sharp reduction in: