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Per the lock compatibility matrix, which combination is allowed concurrently on the same item?
Atwo exclusive locks held by different transactions
Bone exclusive and one shared lock by different transactions
Ctwo shared locks held by different transactions
Dno combination is allowed; one lock per item
Answer & Solution
Correct answer: C. two shared locks held by different transactions
The S-S cell is the only YES in the matrix. Multiple readers can share an item simultaneously. X with anything (S or X) is blocked because the writer needs the room to itself. One-lock-per-item would ban any concurrent reading at all, which is too strict.
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