$(S$&$W12\mathrm{.}5)$ Consider the $IV$ regression model

$Y_i={}_0+{}_1{x}_i+{}_2{W}_i+u_i$

where $x_{i}is$ correlated with $u_i$ and $Z_{i}isan$ instrument. Suppose that the first three

assumptions $in$ Key Concept $12\mathrm{.}4(TheIV$ Regression Assumptions$)$ are satisfied.

Which $IV$ assumption $is$ not satisfied when:

$a{Z}_{i}is$ independent $of(Y_i,x_i,W_i)?$

$b{Z}_i=W_i?$

$c{W}_i=1$ for all $i?$

$d{Z}_i=x_i?$