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  3. Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the bound-aries of effective func

Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the bound-aries of effective func

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问题     Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the bound-aries 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 body 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 blooded" 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】_________.
【C5】
选项 A、they
B、it
C、we
D、what
答案B
解析 形式主语。空格的词要作that引导从句的形式主语。选项中，只有it能作形式主语。故答案为B。
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