A scientist has been tracking and studying a population of deer in Yellowstone National Park. He surveys the population every six months. Usually, the population is thriving and has a gene pool with a wide variety of traits. But during one six-month period, an event occurred that affected the population, and when the scientist returned, the amount of genetic variation had decreased significantly. Which of the following is most likely the event that occurred to change the population

a . Yellowstone National Park expanded its borders.
b . A population of wolves was introduced into Yellowstone National Park.
c . The amount of disease in the deer population decreased.
d . More grass and shrubs were planted in Yellowstone National Park.