Problem $9$

Consider the DAG below. The causal effect of interest is X $->$ Y$.$

$9$A$)$ List all of the open backdoor paths from X $->$ Y$.$

$9$B$)$ What is the minimum number of variables sufficient to control for confounding?

$9$C$)$ In the DAG above, what do we call variable Z$4$ in relation to variables X and Y$?$

$9$D$)$ In the DAG above, what do we call variable Z$3$ in relation to X and Z$1?$