It is pretty well understood controls the flow of carbon dioxide in and out the atmospher

问题补充：

It is pretty well understood controls the flow of carbon dioxide in and out the atmosphere today.

A. that

B. when

C. what

D. how

答案：

A. that

B. when

(答案→)C. what

D. how

解析：分析句子结构可知：句子属于形式主语格式；另外主句中缺少主语。双重作用的引导词只能是what。