【单选题】

The author refers to "external mechanisms" primarily in order to
A．(A) suggest that a biochemical analysis of dinosaur thermoregulation might be a waste of time

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