Give context$-$free grammars that produce the following languages $$over the alphabet $\backslash$Sigma $=\{$a$,$ b$,$ c$\}$: $($a$)$ L$1$ :$=\{$anbm $|$m$<=$n$<=2$m$\}.$

$($b$)$ L$2$ :$=\{$ucv $|$ u$,$v in $\{$a$,$b$\},|$u$|=|$v$|\}.($c$)$ L$3$ :$=\{$aibjck $|$i$=$j ori$=$k$\}$

$($d$)$ L$4$ :$=$