請依下文回答第 36 題至第 40 題： Throughout the history of life, there have been many major upheavals in which whole groups of animals were replaced by others. Perhaps the most famous was the replacement of dinosaurs by mammals about 65 million years ago. Another major change occurred roughly 150 million years earlier, when dinosaurs assumed dominance that had been held for about 80 million years by mammal-like reptiles. What triggered these great upheavals? Scientific thinking about the factors responsible for these replacements has itself changed with significant shifts. At times, scientists suggested that mammals caused dinosaur extinction by eating eggs or competing for the same food resources. However, most researchers now agree that mammals played at most a minor role—since both groups coexisted for millions of years—and that environmental change was primarily responsible. Mammals apparently remained small and inconspicuous in the Mesozoic undergrowth for some 150 million years, until the extinction of dinosaurs gave them the opportunity to radiate into available niches.