Which of the following is a valid SQL statement?
a. SELECT customer#, order#, MAX (shipdate-orderdate)

FROM orders
GROUP BY customer#
WHERE customer# = 1001;

b. SELECT customer#, COUNT (order#)

FROM orders
GROUP BY customer#;

c. SELECT customer#, COUNT (order#)

FROM orders
GROUP BY COUNT (order#);

d. SELECT customer#, COUNT (order#)

FROM orders
GROUP BY order#;