Which of the following is NOT mentioned as a factor of accidents

A．external (外部的) conditions
B．people's internal (内部的) conditions influenced by physical or mental factors
C．slippery (滑的) rod&
D．a chain of events

A．safety regulations effectively prevent the accidents at work
B．with all the safety precautions, accidents at work cannot be avoided
C．the human error or misjudgment leading to accidents at work is mainly due to working environment
D．both B and C