单项选择题
A．conditions in the work place B．the freedom of industries in the past C．changes in industrial production D．the safety and health of workers and customers
A．were put forward due to scientists' recommendations B．came into being as a result of the workers' demand C．were introduced because quite a number of people were killed or seriously injured D．were effective enough to protect workers and customers
A．it senses a coming danger B．it is forced to swim on a hot day C．it is short of its favorite food D．it wants to keep itself to itself