Formulate a constraint to implement the conditions described in each of the following statements.

$($a$)$ Out of projects $2,4,5,$ and $6,$ CRT$\text{'}$s management wants to select exactly two projects.

$($b$)$ Project $1$ can be selected only if project $3$ is selected and vice$-$versa.

$($c$)$ Project $5$ cannot be undertaken unless both projects $2$ and $3$ are also undertaken. $($Only use coefficients of $2,1,1,$ or $2.)$

$($d$)$ If projects $4$ and $5$ are undertaken, then project $6$ must also be undertaken. $($Only use coefficients of $1$ or $1.)$