What is wrong with the following program?

int add $($ int a $,$ int b $)$;

int main $()\{$

int x $=1$;

int y $=5$;

return add $($x $,$ y $)$;

$\}$

A$.$ add$()$ has no definition.

B$.$ add$()$ is called with x and y$,$ but accepts a and b$.$

C$.$ The program does not compile because main$()$ cannot return a value of $6.$

D$.$ A function cannot be called after a return statement.

E$.$ Nothing is wrong with this program