Which ORDER BY clause are valid for the above query?

Evaluate the following SQL statement:
SQL> SELECT cust_id, cust_last_name "Last Name"
FROM customers
WHERE country_id = 10
UNION
SELECT cust_id CUST_NO, cust_last_name
FROM customers
WHERE country_id = 30;
Which ORDER BY clause are valid for the above query? (Choose all that apply.)
A. ORDER BY 2,1
B. ORDER BY CUST_NO
C. ORDER BY 2,cust_id
D. ORDER BY "CUST_NO"
E. ORDER BY "Last Name"

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