艾滋病的传染源是（）
A.蚊
B.猪
C.鼠
D.人
E.螨
参考答案：D
He tried his best to save the _____ boy, but the boy was _____ finally. [ ]
A. drown; drowning
B. drowning; drown
C. drowned; drowning
D. drowning; drowned
膜脂的基本成分是（）
A.胆固醇
B.脑苷脂
C.糖脂
D.鞘磷脂
E.磷脂
