The doctor is skilled at treating heart trouble and never accepts any gift from his patients,so he has a very good_____．
A：expectation
B：reputation
C：contribution
D：civilization
细胞分裂过程中，细胞内变化最明显的是（ ）
已知a、b分别为同周期的ⅠA和ⅦA族的两种元素，它们的原子序数分别为m和n，则下列关系不可能成立的是（ ）
