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