Read an Academic Passage：问题类型与策略

In parts of Central America, certain acacia trees have an unusual relationship with ants. The trees grow large hollow thorns, in which ant colonies live, and produce sugary nectar and protein-rich structures on their leaves that the ants eat. In return, the ants attack insects that feed on the tree and cut back vines and other plants that grow too close to it. For a long time, it was unclear how much the trees really gained from this arrangement. To find out, researchers removed the ants from a group of young trees and compared them with trees whose colonies were left in place. Within a year, the trees without ants had suffered far more insect damage, were often surrounded by competing plants, and grew more slowly. Many of them died. The experiment suggested that, for these acacias, the ants are not simply guests but an essential form of defense. 1. According to the passage, how do the acacia trees benefit from the ants? (A) The ants help the trees grow larger thorns. (B) The ants protect the trees from insects and competing plants. (C) The ants carry the trees' seeds to new areas. (D) The ants provide the trees with nectar and protein. 2. Why does the author describe the experiment in the second paragraph? (A) To show that ants can survive without acacia trees (B) To argue that all trees depend on insects for defense (C) To provide evidence that the trees depend on the ants (D) To explain how researchers remove ants from trees