# Quiz

+ What are the following transactions about TiDB that are incorrect?
  + `A` . Write skew can appear in pessimistic lock mode or optimistic locking mode
  + `B` . Write skew can occur at repeatable read isolation level or commit read isolation level
  + `C` . Is it possible to read different values at the repeatable isolation level using for update
  + `D` . Pessimistic lock mode blocks by default when a lock conflict is encountered

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B