Which occurrence most likely led to the other

three?

(1) Human population growth reached 6.8 billion

in 2010 and it continues to increase.

(2) The number of African elephants has

declined from 1.2 million in 1979 to about

20,000 today.

(3) Approximately 6,500 gallons of oil were

spilled into a river in Illinois after a pipeline

broke.

(4) At one time, rain forests covered 14 percent

of Earth and today they cover only 6 percent.