$(3\mathrm{.}5$ points$)$ Section $5\mathrm{.}1$ Question $I(a,b,c)$

Find context$-$free grammars for the following languages.

$($a$)L=a^n{b}^n,n$ is even.

$($b $)L=a^n{b}^n,n$ is odd.

$($c$)L=a^n{b}^n,n$ is a multiple of three.