Which of the following file organizations is/are I/O efficient for the scan operation in DBMS?

Which of the following statements about the Two Phase Locking (2PL) protocol is/are TRUE?

The relation schema, Person($\underline{\text{pid}}$, $city$), describes the city of residence for every person uniquely identified by $pid$. The following relational algebra operators are available: selection, projection, cross product, and rename.

To find the list of cities where at least 3 persons reside, using the above operators, the minimum number of cross product operations that must be used is

A functional dependency $F: X \to Y$ is termed as a useful functional dependency if and only if it satisfies all the following three conditions:

• $X$ is not the empty set.
• $Y$ is not the empty set.
• Intersection of $X$ and $Y$ is the empty set.