It is pretty well understood ________ controls the flow of carbon dioxide in and out the atmosphere today．
A.thatB.whenC.whatD.how在线课程C
该题考查it作形式主语，what引导主语从句的用法.what引导主语从句，有词汇意义并在主语从句中作主语；that引导主语从句时只起引导作用，无词汇意义，也不能在从句中作任何成分；when表示时间，在主语从句中作状语；how表示方式，在主语从句中作方式状语.句意是：今天，什么控制二氧化碳在大气内外流动已被人们很好地理解.答案为C.
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