$($d$)$ None

$9.$ The magnetic field $B$ at a distance $r$ from a long, straight conductor carrying a current I is

$($a$){}_0\frac{\text{I}}{2}r$

$($b$)\frac{{}_0}{\frac{?}{?}}2r$

$($c$){}_0\frac{\text{I}}{4}r$

$($d$)$ None