Question 19: (2 points) Someone in Registrar's Office complained that he obtained different re- sults when repeating the same query inside one transaction: Tl: SELECT AVG (grade) FROM Enroll; SELECT AVG (grade) FROM Enroll; COMMIT; Q1 -- Q2 Which one of the following actions is sufficient and necessary to ensure that Q1 and Q2 always return the same result? (A) Make all transactions, including T1, run at SERIALIZABLE (B) Make all transactions, including T1, run at REPEATABLE READ (C) Make T1 run at SERIALIZABLE (D) Make T1 run at REPEATABLE READ