The diode cut in voltage is $V=0\mathrm{.}7V$ in the four sircuits shown. Shd $1$ and V in each of the circuits. Assume that the reverse$-$saturation current is $1\mathrm{.}210m$ for each diode.

$($a$)$

$($b$)$

$($c$)$

$($d$)$