Mass deaths of birds were not unheard of, but they normally happened unnoticed at sea or in rural areas away from towns. Many birds are weak and die in winter _______ food is scarce.

A．which

B．what

C．when

D．how

患者，男，58岁，突然剧烈头痛伴恶心、呕吐，脑膜刺激征阳性，脑脊液检查为血性，压力增高，最可能的诊断为（）

A.偏头痛

B.脑血栓形成

C.颅内感染

D.蛛网膜下腔出血

E.三叉神经痛