题目

 Cathy was so careless that she _____ her right arm when she practiced volleyball.    A. hurts              B. hurt              C. has hurt             D. had hurt 答案：B人体的神经细胞与肌肉细胞在形态上完全不一样，这主要是       （   ）A．细胞不断分裂的结果    B．细胞分化的结果    C．细胞自身改变的结果    D．细胞生长的结果