判断下列有关化学基本概念的依据正确的是

A．单质：是否只含一种原子

B．氧化还原反应：元素化合价是否变化

C．共价化合物：是否含有共价键

D．强电解质：熔融状态下是否完全电离

The children _________ for the accident. After all, they were so young.

A．should not be to blame

B．not to be blamed

C．were not to blame

D．were to not blame