Which of the Following Describes the Major Utility of a Eukaryotic

Which of the following describes the major utility of a eukaryotic cDNA library if one is trying to express a protein present in a eukaryotic cell?

A) the library represents all genes that were actively expressed at a specific time
B) the genes lack introns and thus can be expressed by bacterial expression systems
C) cDNA libraries are readily available
D) all of the above are correct
E) both A and B are correct