Problem Description

Consider two transactions, T1 and T2, executing concurrently and accessing the same data item, A. Their operations occur in the following chronological order:

1. T1 reads the initial value of A, which is 100.
2. T2 also reads the initial value of A, which is 100.
3. T1 performs a write operation, updating A to A + 10. Conceptually, T1 intends to write 110.
4. T2 performs a write operation, updating A to A - 10. T2 uses its previously read value of 100, so it conceptually intends to write 90.

Given this sequence of operations, which description best characterizes the concurrency problem observed?

Options

• A. This operation cannot be repeatably read (non-repeatable read).
• B. This operation has no problems.
• C. This operation reads dirty data.
• D. This operation loses modifications (lost update).