Which statement best describes how selective breeding affects the genetic makeup of purebred dog populations? A. Purebred dog breeds have higher rates of genetic mutation than mixed breeds, so genetic diversity increases. B. Desirable characteristics, such as soft fur, become more common in purebred dogs, so genetic diversity increases. C. All harmful traits are eliminated from purebred dog breeds, so genetic diversity decreases. D. Dogs exhibiting desirable characteristics are bred only with one another, so genetic diversity decreases.