In my post about English strategy, which you can read here, I mentioned that you should not skip along merrily from one underlined portion to another. Part of the reason for that is that you are giving yourself little chance to do more than guess on the questions at the end of the passage that are more comprehensive, or require having read the entire passage.
Another reason is that many questions require greater context outside of the sentence at hand (verb tense and pronoun use, being two examples). Check out this example from a recent ACT:
The conjunctive adjective or transition, “Therefore,” is underlined. But such a word serves as a bridge, in a kind of cause and effect. The student who hyper-focuses on the underlined portion may think the way it is written is just fine, and quickly circle “No Change.” However, knowing which word or phrase is best to begin the sentence requires a greater context. The astute student will know that previous sentences dictate that the word “Eventually” is a better beginning in that it transitions from previous sentences to this one.
Let’s focus more now in a more micro way. This is to be stressed: always read the underlined portion in the context of the entire sentence. This is even true (or maybe especially so) when only one word, like the first word in a sentence, is underlined.
Too quick of a glance may result in a hasty conclusion, that this is a fine way to begin a sentence. Or, a quick glance at answer choice “G” may give the impression that the question at stake is whether or not it is proper to put a comma after the word “Observers.” However, if you read the entire sentence, and thus the underlined portion in context, it will sound odd: it is incomplete. We need a choice that creates a complete sentence, which is why the answer is H.
The moral of the story is simple. Beyond reasons of comprehension (which are important), context is crucial for a lot of ACT English questions. Read the entire sentence before answering!