He insisted ________ me with the handbag, though I told him I could manage it myself. [ ]

A. help  

B. in helping

C. on helping  

D. to help

下列关于DNA双螺旋模型的叙述，错误的是

A．是DNA的二级结构
B．双股链相互平行、走向相反
C．碱基位于双螺旋外侧
D．碱基间以非共价键相连
E．磷酸与脱氧核糖组成了双螺旋的骨架