SQL problem. Please answer if youre 100% sure, and answer all parts

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Problem1 Consider a database consisting of a single relation M: 10 a) The following two transactions T1 and T2 run concurrently on this database: T1 BEGIN TRANSACTION S1: update M set B B-10 where A-1 S2: update M set B = B+10 where A=2 COMMIT T2 BEGIN TRANSACTION S3: select sum(B) from M COMMIT Indicate at least one isolation level (if exist) that may make possible to see zero as output for T2. b) The following two transactions run concurrently on this database: T1 BEGIN TRANSACTION S1: insert into M values (3,100) COMMIT T2 BEGIN TRANSACTION S2: select sum(B) from M S3: select sum(B) from M; COMMIT For what isolation levels, T2 may see a different value as the output of the select sum(B) from M statements