Two transactions `T1` and `T2` run concurrently and access the same data item `A`. Their operations (in order of time) are: 1. `T1` reads `A = 100`. 2. `T2` reads `A = 100`. 3. `T1` writes `A = A + 10` (so conceptually writes `110`). 4. `T2` writes `A = A - 10` (using its previously read value `100`, so writes `90`). Which description best characterizes the concurrency problem here? 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).