$(5$ points$)$ Section $1\mathrm{.}2$ Question $14($Grammars$)$:

Find grammars for $=\{a,b\}$ that generate the sets of

$($a$)**$ all strings with exactly two $a\text{'}$s$.$

$($b$)$ all strings with at least two $a\text{'}$s$.$

$($c$)$ all strings with no more than three $a\text{'}$s$.$