患者突发发绀，吸氧不能改善，静脉血呈深棕色，亚甲蓝可使发绀消退，应考虑（）

A.心源性发绀

B.肺源性发绀

C.混合性发绀

D.亚硝酸盐中毒

E.一氧化碳中毒

A man was sighted as he fell overboard. After completing a Williamson turn，the man is not sighted. What type of search should be conducted?（）

A.Expanding circle

B.Sector search

C.Parallel track pattern

D.Datum-drift search