What does '$A$' represent in the given figure?

During the replication of a retrovirus,what is introduced into the host cell?

Which of the following cells are infected in a person suffering from $AIDS$?

$HIV$ causes a reduction in which of the following?

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

Which of the following does $NOT$ spread $AIDS$?