The living things die because there is no ______ in the river or sea.

A．to put oxygen into the river
B．to realize the serious situation clearly
C．to make the waste materials harmless
D．to make special room in the sea for our waste

A．when poisons are poured into water
B．when a great many fish and birds die
C．when the first organisms are affected
D．as soon as the balance of nature is destroyed