BEGIN TRANSACTION SET ISOLATION LEVEL READ COMMITTED select sum(x) from R; select sum(x) from R; select sum(x) from R; commit; Suppose T2 is: BEGIN TRANSACTION SET ISOLATION LEVEL SERIALIZABLE insert into R values (10); insert into R values (20); insert into R values (30); commit; Before any of these transactions execute, the sum of the integers in R is 1000, and none of the integers are 10, 20, or 30. What are possible values that T1 can return