GRE考试

【单选题】Hitherto, there is no evidence yet that would definitively prove whether or not some dinosaurs were warm-blooded, but scientists have begun to investigate　oxygen isotopic abundances in the annual growth bands of teeth or bones of high Line latitude, hence seasonally influenced, terrestrial dinosaurs. The ratio of oxygen isotopes depends on temperature, and an absence of seasonal variations in oxygen 18―a heavy version of the common oxygen 16 atom―would　strongly suggest that the animals maintained a constant internal　temperatures. Such a finding would not, however, constitute "proof"　that dinosaurs were warm blooded, as there are external mechanisms that cold-blooded animals employ to　regulate body temperatures and thereby influence metabolic rates. More nettlesome is that proof would have to come from discovery of intact dinosaur remains in which the soft tissue had not been replaced or altered, and from which the biomolecules responsible for thermoregulation could be extracted identified and characterizeD、Such a proof is unlikely, as it would require an　almost impossible level of preservation over 65 million years, plus the advent of biotechnology that does not yet exist. According to the passage, the most severe problem currently facing scientists in conclusively determining whether certain dinosaurs were warm-blooded is________.
A、proving that thermoregulation is the primary means by which metabolic rates are influenced
B．developing a way of distinguishing between oxygen 18 and oxygen 16
C．showing that an absence of seasonal variation in oxygen 16 implies a constant internal temperature
D．finding a sample of dinosaur tissue which has survived 65 million years in a relatively intact form
E．determining which species of dinosaurs were most likely to be warm-blooded, and focusing research there

网考网参考答案：D
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