OG About 5 million acres

This is one of the hardest questions in the OG, mostly because of the difficulty in choosing between A and B.

In choice A, the fact that the dependent clause "that gives mouth sores to cattle" modifies "milky sap" is not enough to disqualify this choice. In fact (outside knowledge from an ecologist), the milky sap contains ingenol, a toxic substance that indeed causes both mouth sores and gastric lesions in cattle. Of course the student is not expected to know this. The point is this: there is nothing in the wording of choice A that betrays this placement of the modifying clause "that gives mouth sores to cattle" as wrong or illogical. What is suspicious, however, is this: while this modifying clause modifies "milky sap", the modifying participial phrases beginning with "giving ..." and "displacing ..." must logically modify "a herbaceous plant". While we don't expect strict parallelism between a modifying clause and separate modifying phrases, given the complexity of the sentence it is highly desirable. There is a certain ambiguity as to what the participial phrases modify.

Choice B places "with milky sap" between commas, as unnecessary information, which means that the dependent clause "that gives mouth sores to cattle" now modifies "a herbaceous plant". This alone is neither better nor worse than the arrangement in choice A--the reassignment of that modifying clause is a little less precise but not out-and-out wrong. The placement of "with milky sap" between commas is a bit clunky but not grammatically offensive. So, at this point, both A and B are basically solid but each has a couple of dings against it.

So here's a case where the best approach is to focus on the main idea and supporting ideas about "leafy spurge" in each answer choice. In choice A, the main idea is that leafy spurge is "a herbaceous plant ... with sap ... that gives mouth sores". Then come the additional facts, expressed by parallel participial phrases, that this plant "[displaces] grasses and other ... food" and [renders] rangeland worthless." This is fine as it is, but choice B tells a more coherent story and for that reason is to be preferred. But you must compare them directly in order to discern why.

In choice A, as we have seen, there is one main fact about leafy spurge (actually, its sap) and two additional facts. In choice B, there are two main facts and only one additional fact. The main facts are that this "herbaceous plant ... gives mouth sores to cattle and displaces grasses and other ... food". The additional fact is that the plant "[renders] rangeland worthless". This tells a more coherent story. Leafy spurge has two immediate physical effects on cattle (giving mouth sores) and their food source (displacing grasses); and through these effects it brings about the less tangible result of rendering rangeland worthless.

Choice B is less precise and has some clunky wording. However, the parallelism in choice B is superior grammatically--both the modifying clause AND the modifying participial phrase modify the same thing, "a herbaceous plant"; and the parallelism also makes better sense and offers a more coherent story. It would probably take me longer to tease out the rather minor and unusual error in parallelism in choice A, but less time to figure out that B does a better job of telling the story.

One error in this sentence occurs in all the answer choices. In American English "herbaceous" would be preceded by "an", not by "a".