Question

The Scottish chemist Thomas Graham observed and experimented with gas particles in the 1800s. He formulated a mathematical equation that describes the relationship between the mass of a gas molecule and its rate of effusion. This relationship is a

A. law because it describes what happens without explaining why.
B. law because it is based on mathematics.
C. theory because it describes a relationship.
D. theory because it is a description without an explanation.

i'm really not sure, I think it's law but then for this question i'm thinking its D? Can someone help me?