Why would researchers want to have both genomic libraries and cDNA libraries from a particular organism?

A) because a cDNA library helps us understand how retroviruses work, but a genomic library does not
B) because a genomic library is generally too big to be helpful, while a cDNA library, being smaller, is much easier to study
C) because a genomic library will be the same in every cell, while a cDNA library tells researchers which genes are being expressed in individual cell types
D) because a cDNA library is a lot easier to clone than a genomic library