Question

Which pair of variables shows a clear causal link?

Answer 1

The number of lights switched on in a room and the room's brightness.

Answer 2

Answer:

The pair of variables that show a clear causal link are  the number of lights switched on in a room and the room's brightness

Step-by-step explanation:

Clear casual link indicates two variables that have a completly and unequivocal relation, meaning that changes in one variable produce automatically and with no doubt a change in the other.

Case A does not have this link as these two variables have no relation at all. A person from certian state may have any type of academic degree.

Case B Cost of aviation fuel  may  be explained by other variables like international prices and flight duration may be explained also due to climate.

Case D- a person may have a pet even under low income levels.

Case C is the only adequate relation, as turning lights on will always increase room's brightness.