对于上消化道大出血，经过恰当治疗，临床上出现以下哪些情况考虑存在继续出血或再出血（）

A.成型黑便

B.经充分补液及输血后血红蛋白浓度、红细胞计数继续下降，网织红细胞持续增高

C.周围循环衰竭经充分补液及输血后好转而又恶化

D.补液与尿量足够的情况下，血尿素氮持续或再次增高

E.咳嗽加剧

Up to now, no one has come up with a（n）       explanation of why dinosaurs died out, and it remains a mystery．

A．astonishing

B．amusing

C．convincing

D．puzzling