Final answer:
A cDNA library would be used to study the promoter of a gene because it contains DNA from only the expressed genes in the cell, excluding introns, control sequences, and non-translated DNA. On the other hand, a genomic library contains genomic fragments and is not suitable for studying the promoter of a specific gene.
Step-by-step explanation:
A cDNA library would be used to study the promoter of a gene. A cDNA library is constructed using the organism's messenger RNA (mRNA) and contains DNA from only the expressed genes in the cell. This means that the library does not include introns, control sequences such as promoters, and DNA that is not translated into proteins. On the other hand, a genomic library contains genomic fragments and is not suitable for studying the promoter of a specific gene.