The expression $\{S\mid \neg(S\in \mathrm{Sailors})\}$ asks for every possible tuple $S$ that is not present in the $\mathrm{Sailors}$ relation.
The problem is that $S$ is not restricted to tuples that come from any relation in the database.
It can therefore range over tuples outside the active domain of the database.
This can produce an unbounded result, so the TRC expression is unsafe.
Ordinary relational algebra works only with tuples obtained from the relations in the database.
It does not have an operation for taking the complement of a relation with respect to the set of all possible tuples.
Therefore, there is no equivalent ordinary relational-algebra expression.
Answer : A
