How can microarrays help us understand cellular functions?

A) Microarrays let us study different cells under different conditions.
B) Microarrays let us directly measure protein expression in individual cells.
C) Microarrays, by hybridizing cDNAs to DNA sequences already present on a chip, let us identify which portions of a genome are being expressed in a cell at a particular time.
D) Microarrays let us identify which DNA sequences are present in a particular cell type under certain conditions.