TOEFL Reading: ETS-TOEFL阅读机经 - Z5J1HGD6S35ZB755L$

In paragraph 5, why does the author discuss experiments in which some burrows were fumigated? A. To demonstrate that parasites have a very negative effect on large colonies B. To show that attacks by predators are a worse problem than the spread of parasites C. To explain how swallows inspect nests for parasites D. To prove that the benefits of colonial nesting outweigh the disadvantages