How is CIDR $/21$ written in binary $($format $$

Choose one $*1$ point

$11111111\mathrm{.}00000000\mathrm{.}00000000\mathrm{.}0$

$0000000$

$11111111\mathrm{.}11111111\mathrm{.}11000000\mathrm{.}0$

$0000000$

$11111111\mathrm{.}11111111\mathrm{.}11111000\mathrm{.}0$

$0000000$

$11111111\mathrm{.}11111111\mathrm{.}11111110\mathrm{.}0$

$0000000$