$1\mathrm{.}18$ Perform subtraction on the given unsigned binary numbers using the $2\text{'}$s complement of the subtrahend. Where the result should be negative, find its $2\text{'}$s complement and affix a minus sign.

$($a$)10011-10010$

$($b$)100010-100110$

$($c$)1001-110101$

$($d$)101000-10101$