问题补充：
--- What’s your question?
---I want to know .
A. how can trees help us
B. how do trees help us
C. how trees help us
D. how can we help trees
答案：
A. how can trees help us
B. how do trees help us
(答案→)C. how trees help us
D. how can we help trees
解析：根据题干，我们可知本句是一个宾语从句或间接引语。宾语从句或间接引语的语序应该用陈述句语序，所谓陈述句语序，就是谓语动词（包括be动词，助动词和情态动词）应放在主语之后。通过观察，只有C是陈述句语序。所以本题选C。
