Which of the Following Is an Example of Reverse Genetics

Which of the following is an example of reverse genetics?

A) A geneticist maps the location of mutations associated with strains of mice found to have heart defects.
B) A geneticist searches for naturally occurring mutations that affect the sleep cycle in breeds of dogs.
C) A geneticist runs a Southern blot to map the location of a mutant gene in a species of bacterium growing in a hot spring and then clones that mutant gene.
D) A geneticist experimentally creates a mutation in a gene of unknown function in Drosophila and then observes the mutant phenotype.
E) None of these is an example of reverse genetics.