大圆的半径是8厘米,小圆的直径是8厘米。大圆与小圆的面积比是[ ]
A.1:1
B.2:1
C.4:1
D.3:2
答案:C
Li Lei often helps me _____ English in the evening.[ ]
A. study
B. studies
B. studying
D. studys
将混合生物碱溶于氯仿中,用pH由高到低的酸性缓冲溶液顺次萃取,生物碱则可按下列顺序依次萃取出来
A.碱度由弱到强
B.碱度由强到弱
C.极性由小到大
D.极性由大到小
E.酸度由弱到强