Which statements will be able to show the following result set?

a$.)$

SELECT invoice$\_$id$,$ invoice$\_$date, email

FROM invoice

WHERE customer$\_$if $!=$ NULL;

b$.)$

SELECT invoice$\_$id$,$ invoice$\_$date, email

FROM invoice, customer;

c$.)$

SELECT invoice$\_$id$,$ invoice$\_$date, email

FROM invoice

USING $($invoice$.$customer$\_$id $=$ customer.customer$\_$id$)$;

d$.)$

SELECT invoice$\_$id$,$ invoice$\_$date, email

FROM invoice

JOIN customer

USING $($customer$\_$id$)$;