A. We should know what would happen in the future. B. We should plant more trees and flowers. C. We should clean the banks of our rivers. D. We should know we should do and begin to do it now. 参考答案:A 答案解析:[分析] 本题测试点为对文章主旨的掌握。题目提出:“对我们来说要保护世界最重要的事是什么 ”根据文章的内容当然“首先要明白现在不保护自然将来会发生什么”最重要。故选A。