Find grammars for $Σ$ = {$a, b$} that generate the sets of
(a) all strings with exactly one $a$.
(b) all strings with at least one $a$.
(c) all strings with no more than three $a$’s.
(d) all strings with at least three $a$’s.
In each case, give convincing arguments that the grammar you give does indeed generate the
indicated language.
