@victoriac, (E) actually doesn't resolve anything but rather adds more to the mystery. You're trying to figure out why dry grass clippings added to soil would benefit plants while fresh grass clippings added to soil hurt plants. Saying that adding both hurts plants doesn't resolve the seeming conflict.
(D) however, provides a possible explanation for the seeming paradox. The original statement says that dry grass clippings decompose slowly and feed helpful bacteria - improving plant growth. If fresh grass clippings decompose quickly and create high levels of heat that kill the helpful bacteria, then this could explain why plant growth suffers, and you've solved your seeming paradox.
