【单选题】

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 (31) around 37℃; a (32) , of a few degrees from this value seriously impairs their functioning.Even though cells can survive wider (33) , the integrated actions of bodily systems are impaireD、Other animals have a wider tolerance (34) changes of bodily temperature.

For centuries (35) has been recognized that mammals and birds differ from other animals in the way they regulate body temperature. Ways of (36) the difference have become more accurate and meaningful over time, but popular terminology still reflects the old division (37) " warm blooded" and "cold blood- ed " species; warm blooded included mammals and birds, (38) all other creatures were considered cold bloodeD、 (39) more species were studied, it became evident that this classification was (40) A、fence lizard usually has a body temperature only a degree or two below (41) of humans and so is not colD、 (42) the next distinction was made between animals that maintain a (43) body temperature and those whose body temperature varies with their environment.But this classification also proved inadequate, (44) among mammals there are many that vary their body temperatures during hibernation. (45) many invertebrates that live in the (46) 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 (47) chiefly by internal metabolic processes and those whose temperature is regulated by the (48) . The latter do so mainly by moving to favorable sites or by changing their exposure to (49) 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 (50) .
A．With
B．Like
C．As
D．Being