将外毒素用0.4%甲醛处理为无毒且保留抗原性的物质称为（）

A.抗毒素

B.类毒素

C.减毒活疫苗

D.死疫苗

E.丙种球蛋白

When launching a lifeboat in the rough sea, the davit gears should be released（）.

A.before the boat is lowered from the stowed position

B.as the boat fall blocks break clear of the davit head

C.before the boat is lowered from the embarkation level

D.after the boat is released into the water