Dried grass clippings mixed into garden soil gradually decompose, providing nutrients for beneficial soil bacteria. This results in better-than-average plant growth. Yet mixing fresh grass clippings into garden soil usually causes poorer-than-average plant growth. Which one of the following, if true, most helps to explain the difference in plant growth described above? ['When a mix of fresh and dried grass clippings is mixed into garden soil, plant growth often decreases.', 'The number of beneficial soil bacteria increases whenever any kind of plant material is mixed into garden soil.', 'Some dried grass clippings retain nutrients originally derived from commercial lawn fertilizers, and thus provide additional enrichment to the soil.', 'Fresh grass clippings mixed into soil decompose rapidly, generating high levels of heat that kill beneficial soil bacteria.']