Given the struct declaration:

struct exp $\{$

explicit exp$($char const x$)\{\}$

$\}$;

And the helper function:

void fn$($exp x$)\{\}$

Which of the following statements is not formed properly?

$1.$

exp y exp$("$ok$")$;

$2.$

exp x $="$ok$"$;

$3.$

fn$($exp$("$ok$"))$;

$4.$

fn$("$ok$")$;

$5.$

exp z$("$ok$")$;