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