_____ a cold morning, I opened the window and was glad to find that it was snowing outside. [ ]

A. By

B. On

C. In

D. At

中毒型痢疾的发病原理是（）

A.机体免疫力下降

B.痢疾杆菌毒力过强

C.感染痢疾杆菌的数量过多

D.机体对细菌毒素异常强烈的反映

E.局部IgA分泌不足