For a given alphabet $\backslash$Sigma $=\{$p$,$ q$,$ r$\},$ assume there is a regular language L representing all strings that use at most two of the three letters in this alphabet. Examples of valid strings include rprpp$,$ p$,$ qqq$,$ pqqqp$,$ and the empty string. An example of an invalid string is pqrpr$,$ since it contains all three letters.

$($a$)$ Create a regular expression for L$.$

$($b$)$ Draw an NFA for L$.$

$($c$)$ Draw a DFA for L$.$