单选题 The author refers to "external mechanisms" primarily in order to

• (A) suggest that a biochemical analysis of dinosaur thermoregulation might be a waste of time
• (B) suggest that other means than warm-bloodedness may account for evidence of dinosaur thermoregulation
• (C) explain why scientists are interested in looking for an absence of seasonal variations in oxygen 18 traces
• (D) suggest that metabolic rates, not warm-bloodedness, may have been the key factor in certain dinosaurs' survival
• (E) imply that other factors beside temperature may dictate the ratio of oxygen isotopes