In Discussing Stable Isotopes, Which Are Used to Study Global $\delta$

In discussing stable isotopes, which are used to study global chemical cycles, the symbol $\delta$ is used to relate the isotopic composition of

A) istopically heavy to istopically light samples.
B) istopically light to istopically heavy samples.
C) unknown samples to a standard sample to which all others are compared.
D) unknown samples to the average of all samples for the study at hand.