在光合作用實驗里,如果所用的水中有0.44%的水分子含18О,二氧化碳中有0.68%的二氧化碳分子含18О。那么,植物進行光合作用釋放的氧氣中,含18О的比例為-----(     )
A.0.20%B.0.44%C.0.68%D.0.88%
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