Write the regular expression for the following language D$.$ The alphabet is $\backslash$Sigma $=\{$a$,$b$,$c$,$d$,$e$,$ f $,$g$,$h$,$ i$,$ j$,$k$,$ l$,$m$,$n$,$o$,$ p$,$q$,$ r$,$ s$,$ t$,$u$,$v$,$w$,$x$,$y$,$ z$\}.$ D $=\{$w$|$ w is a string with the number of a$\text{'}$s divisible by $3\}$