The key description that we have for Bill is his "crazy" lifestyle. In contrast to this, he has decided at last to "settle down," meaning that he has decided to live a calmer life. When one is "sensible," that person is practical and prudent. A "sensible" set of actions is one that is level-headed—far indeed from being "crazy."

The implied contrast in this sentence is between "optional" and our blank. The word "optional" means "not necessary." When you have many options, it is likely not the case that any one of them must be chosen. In contrast to such "optional" things, the cleaning must be completely "necessary" for Laura. Therefore, this is the best option. Even "joyful" is not good enough, for we do not know whether or not cleaning is a source of joy for her. We only know that it must be done.

A mystery requires a lot of attention paid to all the details involved. Therefore, when you "probe" (that is, look into) them, you must consider them very carefully. The adverb "thoroughly" best fits this description. When something is "thorough", it is complete—as though it "goes through" everything. A "thorough investigation" is one that sifts through all the details with great care and depth.

The sentence opens by explaining the natural state of human life. Most people need seven to eight hours of sleep (at least according to this sentence). The word "although" indicates a contrast between these "normal" humans and those needing less sleep. While we might think that such sleep patterns lead to sickness, we cannot assume this is what the author wishes to express. All we know is that he or she is contrasting normal and abnormal patterns. Therefore, the safest answer is "unnatural."

We cannot be sure how the boundaries of the playground are marked. (Therefore, we cannot choose either "chalked" or "gated.") We do know, however, that the boundaries must appear rather obvious. There is "no doubt" if someone goes too far beyond them. Since there is no doubt, they must be rather "definite", meaning that they are clearly and well defined. Something "indefinite" is lacking such clarity (and is, perhaps, even "nebulous").

In this sentence, "up to date" describes the "ever new" part of the quotation. The "old and respectable" applies to the first part (in which our blank is found). We could say "ever old, ever new," though that is not an option. The word "ancient" means very old. This is the best option. Do not be tempted by "decrepit"; a "decrepit" thing is old and feeble. This does not fit with "respectable."

The key clue for this sentence is the clause starting with "though." Although it would take a long time to count all of the sand on the beach, it is still possible to do so. This means that its grains are not endless in number. Therefore, they are not infinite. They are, instead, "finite." The word comes from Latin for end. Think of other English words like "finale" and "finish." To be "finite" means coming to a finish. A finite thing is not endless.

This sentence contrasts two personalities—that of Christopher and that of Eliza. Eliza likes to store her books in an orderly fashion. It disturbs her that Christopher does not. While he might be immature or annoying in his storage manners, the implied contrast is with "orderly." If someone does something in a random manner, this lacks any apparent order. Therefore, of the options provided, this is the best.

The sentence implies that after a major life event, people can begin to see their friends suddenly again. If any of the options were synonymous with "suddenly," they would be potential correct answers. However, as such an answer is not provided, we need to find something else that contrasts well with not seeing friends for years. In contrast to such extended absence, "frequent" visits are quite different. Therefore, of the options provided, this is the only one that adequately works without importing other meanings (as happens with negative words like "flippantly," "sadly," and "manipulatively").

The conjunction "for" functions much like "because." The second half of the sentence therefore explains the first half. In the second half, it is clear that regularity and routine are helpful for the child. This indicates that he or she would need a parent who lives with such regularity. When we are "consistent," we do not change with regard to our duties or activities. Therefore, this word best describes the behavior of such good parents.