Since the beginning of the twentieth century,the rate at which ______ and other so-called greenhouse gases have entered the atmosphere has been faster than at any other time in the known history of life on Earth.
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The Answer of Since the beginning of the twentieth century,the rate at which ______ and other so-called greenhouse gases have entered the atmosphere has been faster than at any other time in the known history of life on Earth.
or

Since the Beginning of the Twentieth Century,the Rate at Which