$7.$ Given the following query:

SELECT zip, order#

FROM customers NATURAL JOIN orders;

Which of the following queries is equivalent?

a$.$ SELECT zip, order#

FROM customers JOIN orders

WHERE customers.customer# $=$ orders.customer#; b$.$ SELECT zip, order#

FROM customers, orders

WHERE customers.customer# $=$ orders.customer#;

c$.$ SELECT zip, order#

FROM customers, orders

WHERE customers.customer# $=$ orders.customer# $(+)$;

d$.$ none of the above