This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
True about gene library 
Select and explain. 
A. Also known as chromosome, B. Library that contains books on gene, C. Computer base with all gene knowledge, D. DNA nucleotide or fragment
D: DNA nucleotide or fragment
Answer: D: DNA nucleotide or fragment 
Explanation: A DNA library is a set of cloned fragments that collectively represent the genes of a paicular organism. Paicular genes can be isolated from DNA libraries, much as books can be obtained from conventional libraries. The secret is knowing where and how to look. There are two general types of gene library: a genomic library, which consists of the total chromosomal DNA of an organism; and a cDNA library, which represents the mRNA from a cell or tissue at a specific point of time.