CAT Critical Reasoning Practice question with Solution 23

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?

[A]. The number of beneficial soil bacteria increases whenever any kind of plant material is mixed into garden soil.
[B]. Nutrients released by dried grass clippings are immediately available to beneficial soil bacteria.
[C]. Some dried grass clippings retain nutrients originally derived from commercial lawn fertilizers, and thus provide additional enrichment to the soil.
[D]. Fresh grass clippings mixed into soil decompose rapidly, generating high levels of heat that kill beneficial soil bacteria.
[E]. When a mix of fresh and dried grass clippings is mixed into garden soil, plant growth often decreases.
Answer: D

Why would dried grass clippings be better for plant growth than fresh grass clippings? We know that dried clippings decompose gradually, providing nutrients, but the stimulus provides no information about the qualities of fresh grass clippings. Choice (D) solves the mystery. If the rapid decomposition of fresh clipping generates heat that kills beneficial bacteria, then dried clippings, which decompose gradually, would provide better results. (A) and (E), if true, apply equally well to both dried and fresh grass clippings, and so they cannot explain why one would be better than the other.

(B) This is all well and good, and supports the notion expressed in the stimulus that dried grass clippings are good for beneficial soil bacteria and plant growth. However, what this choice fails to do is explain why dried clippings are more effective than fresh clippings, which, for all we know, could have the same qualities.

(C) has the same problem as answer choice (B)—continuing to heap praise on dried grass clippings doesn’t tell us the one thing we want to know: why is this kind of clipping better for plant growth than fresh grass clippings? Stating the benefits of dried grass clippings by itself does not explain the mystery, because for all we know the fresh grass clippings have the same benefits. We need an explicit difference between the two, which choice (C) doesn’t provide. Moreover, (C) applies to some dried clippings, whereas the stimulus applies to dried grass clippings in general.

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