For each of the following context$-$free languages, give a CFG$.$

$($a$).L_1=\{a^i{b}^j$:$i2j\}.$

$($b$).L_2=\{uv$:$u,$vin$\{a,b{\}}^{*},|u|=|v|,uv\}.$

$($c$).L_3=\{b^k{a}^n{b}^n$:$n,k0\}.$