GMAT Critical Reasoning (CR) | Urban air contains more sulfur dioxide than does rural air,

I came up with the assumption : Plant growth rate is not affected by Sulphur Dioxide at all and is affected by some other reason. Choice D gave me that reason and hence I selected it.egmat can you comment if my assumption is correct? I assume is it since I was able to select the right choice and none of the other choices are close to the assumption I came up with.
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