How much sleep does a person need?【B1】the physiological bases of the need for sleep remain conjectural(猜想), rendering conclusive answers to this question impossible, much evidence has been gathered on how much sleep people do in fact obtain. Perhaps the most important conclusion to be【B2】from this evidence is【B3】there is great variability among individuals in total sleep time. For adults,【B4】between six and nine hours of sleep as a nightly average is not unusual, and 7.5 hours probably best expresses the norm. Such norms, of course, forms inevitably vary with the criteria of sleep employed. The most【B5】and reliable figures on sleep time, including those cited here, come from studies in sleep laboratories, where EEG criteria are employed. 【B6】consistently has been associated with the varying amount, quality, and pattern of electrophysiologically defined sleep. The newborn infant may spend an average of about 16 hours of each 24-hour period in sleep,【B7】the sleep time drops sharply; by two years of age, it may【B8】from nine to 12 hours. Decreases to approximately six hours have been observed among the elderly. 【B9】will be discussed from below, EEG sleep studies have indicated that sleep can be considered to consist of several different stages. Developmental changes in the relative proportion of sleep time【B10】in these sleep stages are as striking as age-related changes in total sleep time.
单选题
【B1】
【正确答案】
C
【答案解析】解析：做完形填空题往往需要"瞻前顾后"才能做出判断。本题必须要将句子下文看完才能确定答案。后面的much evidence has been gathered on how much sleep people do in fact obtain与前面提到的"睡眠心理基础并不明确"意思对立，因此选择含有"对比，让步"意思的连词while合理。despite虽然也能表达让步关系，却是介词，不能引导从句。
【答案解析】解析：本题的关键是区别意思相关联的形容词用法。brief表示"简要的"；precise表示"准确的"；correct意思为"正确的"；concise则为"简明的"。句子后半部分come from studies in sleep laboratories表示数字是来自实验室的，因此precise较为合理。