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

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A. that
B. when
(答案→)C. what
D. how
解析：主要测试主语从句。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。例如：It is still doubtful what he said at the meeting yesterday。昨天他在会上说的话仍然值得怀疑。