【单选题】

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