A．factories and cloudsB．cars and oilC．smoke and fumesD．……

Air pollution is caused by many things. Great clouds of smoke come from factories. Cars send smoke and fumes into the air. Burning waste fills the air with bits of dirt. Even burning leaves add smoke to the air. Sometimes, planes cannot find their landing spots through smoke. Smoke of all kinds is bad to breathe and can do great harm to the body.
Still, we cannot live without air. We must find a way to clean the air we have.
In time, we may have factories that are run by atomic energy. Our cars may run on smoke-free electric power. Scientists are also working on new ways to help the oil burned by cars from making fumes. Waste may be burned in the sand on ocean floors.
These changes might keep our air clean in the years to come. But until then, many scientists are looking for ways to make air cleaner now.
Air pollution is caused by ______.

A．factories and clouds
B．cars and oil
C．smoke and fumes
D．waste burned on ocean floors