Why do different kinds of control mechanisms exist?

Cells contain a diverse set of different mechanisms for controlling gene activation. In general, the type of mechanism controlling the synthesis of a gene product depends on the product's function. If a product is involved in an catabolic pathway then gene expression will be stimulated by initial substrates of the pathway and repressed by final products of the pathway. Initial substrate molecules can either inactivate repressor proteins or induce activator proteins. Final product molecules would activate repressor proteins. Using these types of control mechanisms the genes for the pathway molecules could be turned on when there is a lot of the substrate molecules and turned off when there is a lot of the of the final product. V

An active repressor protein:
A. is a structural protein.
B. binds to the DNA molecule and promotes transcription initiation.
C. binds to the DNA molecule and prevents transcription. binds to the RNA polymerase, preventing messenger RNA synthesis.
D. binds to an inducer to activate it.