其实我觉得C/D都对。我选D没选C，是因为C有点废话，而且是不可知的（未来能不能发现，谁知道呢？）。请教一下,C/D该怎么选，谢谢。:-)
附上题目：
607. Birds have been said to be descended fromcertain birdlike dinosaur species with which they share distinctive structural features. The fossilrecord, however, shows that this cannot be so, since there are bird fossils that are much older than the earliestbirdlike dinosaur fossils that have been found.
Which of the following is an assumption on whichthe argument relies?
(A) The birdlike dinosaurs have no livingdescendants.
(B) There are no flightless dinosaur species thathave the distinctive structural features shared by
birds and birdlike dinosaurs.
(C) There are no birdlike dinosaur fossils thatare older than the bird fossils but have not yet been
unearthed.
(D) It could not have been the case that somebirds were descended from one of the birdlike
dinosaur species and other birds from another.
(E) Birds cannot have been descended fromdinosaur species with which the birds do not share the distinctive structuralfeatures.
