选B.
题干有according to the passage.文章中提到原因是Most scientists agree that the earth is getting warmer and that CO2 gases are part of the problem.因为is getting,所以已经在发生了.B选项has been taking place,过去完成进行时,表示过去现在一直在发生,将来很可能继续发生,符合题意.
原文选段：The theory of global warming is no longer a theory.Most
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