Researchers have found that some unprotected areas outside of a national park that was designed to protect birds have substantially higher numbers of certain bird species than comparable areas inside the park. Which one of the following, if true, most helps to explain the researchers' finding? ['The park was designed to protect endangered bird species, but some of the bird species that are present in higher numbers in the unprotected areas are also endangered.', 'The researchers also found that some unprotected areas outside of the park have substantially higher numbers of certain reptile species than comparable areas inside the park.', 'Moose are much more prevalent inside the park, where hunting is prohibited, than outside the park, and moose eat much of the food that the birds need to survive.', 'Both inside the park and just outside of it, there are riverside areas containing willows and other waterside growth that the bird species thrive on.']