## Core Concept
A gene library, also known as a genomic library or cDNA library, is a collection of DNA sequences that represent the genetic material of an organism. This concept is crucial in molecular biology for cloning and studying genes. The library is constructed by fragmenting the DNA into smaller pieces, cloning these fragments into vectors, and then storing these vectors in a collection.

## Why the Correct Answer is Right
The correct answer involves understanding the characteristics and applications of a gene library. A gene library allows researchers to isolate and study specific genes or gene families. It can be made from any organism and can represent the entire genome (genomic library) or only the expressed genes (cDNA library). This tool is essential for understanding gene function, identifying disease-causing genes, and developing gene therapy.

## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific details of option A, we cannot directly address its inaccuracies. However, any option that does not accurately describe a true characteristic or application of a gene library would be considered incorrect.
- **Option B:** Similarly, without specifics, we can't directly refute option B. If it suggests a limitation or feature not applicable to gene libraries, it would be incorrect.
- **Option C:** This option's accuracy depends on its content. If it misrepresents how gene libraries are constructed, their purpose, or their applications, it would be wrong.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **cDNA libraries** are made from mature mRNA and thus only contain sequences that are expressed in the cell or tissue from which the mRNA was isolated. This makes cDNA libraries particularly useful for studying gene expression and for isolating genes that are actively expressed in specific tissues or developmental stages.

## Correct Answer: D.