Recorded history has taught us ______.

A．studies on the cause of cancer
B．controversial view-points in the cause of cancer
C．the relationship between electricity and cancer
D．different ideas about the effect of electricity on caner

A．the force of the electromagnetic field is too weak to be harmful
B．the force of the electromagnetic field is weaker than the electric field that the cells generate
C．electromagnetic field may affect health
D．only more powerful radiation can knock electron out of human body