Q $2$: Write Context Free Grammar for following languages

$($i$)$ A language having all strings of $a^{\text{'}}s$ and $b,s$ with each string like $a^x{b}^y,$ where $x=2y$

$($ii$)$ A language having all strings of even length, over ${\displaystyle \underset{?}{\overset{?}{}}}=\{a,b\}.$

$($iii$)$ A language having all strings over ${\displaystyle \underset{?}{\overset{?}{}}}=\{a,b\}$ with Regular Expression $($ab$)*a$

$($iv$)$ A language of ODD palindromes over ${\displaystyle \underset{?}{\overset{?}{}}}=\{a,b\}.$