托福听力official31 Paragraph 5 supports which of the following statements regarding the importance of fires in maintaining savannas? 答案解析

用importance of fire去定位，对应部分从but in experimental are as protected from fire 开始，后面的内容都对其重要性进行了解释：But in experimental areas protected from fire, a few perennial grass species eventually come to dominate, outcompeting all others. Evidence from other studies suggests that exclusion of fire results in markedly decreased plant-species richness, often with an increase in tree density. 从这两句话可以看到，如果没有火来保持savannas，那么就很少有perennial grass species可以outcompeting其他的种类，从而减少了plant‐species的丰富，而树的密度则会更大。对应了D的描述。ABC选项都在原文未提及。