Write the regular expressions corresponding to the following languages defined

over $=\{$a$,$ b$\}$:

$$ L $=\{$a$,$ aa$,$ ab$\}.$

$$ L is the language of all the strings that contain aa as a substring.

$$ L is the language of all the strings that contain one and only one occurrence

of aa as a substring.

$$ L is the language of all the strings that contain one or more consecutive

repetitions of ab as a prefix.