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  3. Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the boundaries of effective funct

Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the boundaries of effective funct

admin2012-07-22  48
问题      Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the boundaries of effective functioning. Enzyme systems of mammals and birds are most efficient only within a narrow 【C1】______ around 37℃; a 【C2】______  , of a few degrees from this value seriously impairs their functioning. Even though cells can survive wider 【C3】______  , the integrated actions of bodily systems are impaired. Other animals have a wider tolerance 【C4】______  changes of bodily temperature.
     For centuries 【C5】______  has been recognized that mammals and birds differ from other animals in the way they regulate body temperature. Ways of 【C6】______  the difference have become more accurate and meaningful over time, but popular terminology still reflects the old division 【C7】______  " warm blooded" and "cold blood- ed " species; warm blooded included mammals and birds, 【C8】______  all other creatures were considered cold blooded. 【C9】______  more species were studied, it became evident that this classification was 【C10】______  A fence lizard usually has a body temperature only a degree or two below 【C11】______  of humans and so is not cold. 【C12】______  the next distinction was made between animals that maintain a 【C13】______  body temperature and those whose body temperature varies with their environment. But this classification also proved inadequate, 【C14】______  among mammals there are many that vary their body temperatures during hibernation. 【C15】______  many invertebrates that live in the 【C16】______ of the ocean never experience a change in the chill of the deep water, and their body temperatures remain constant. The current distinction is between animals whose body temperature is 【C17】______  chiefly by internal metabolic processes and those whose temperature is regulated by the 【C18】______ . The latter do so mainly by moving to favorable sites or by changing their exposure to 【C19】______  sources of heat. Mammals and birds also regulate their temperature by choosing favorable environments, but primarily they regulate their temperature by making a variety of internal 【C20】______ .
【C12】
选项 A、But
B、However
C、Therefore
D、Meanwhile
答案C
解析 由于前句提出了反例，指出传统的分类有不足之处。本句接着讲到更准确的分类。因此可推断它们之间是因果关系，所以选C。
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