Which of the following SQL statements will return the year and average salary for all employees hired within a given year that have a salary greater than $30,000?（）

• A、SELECT * FROM t1 UNION SELECT * FROM t2
• B、SELECT * FROM t1 UNION DISTINCT SELECT * FROM t2
• C、SELECT * FROM t1 INTERSECT SELECT * FROM t2
• D、SELECT * FROM t1 WHERE (c1,c2)= (SELECT c1,c2 FROM t2)