Which opening sentence goes best with the details in the above paragraph?
A.
Recycling programs have become popular.
B.
Pollution from burning of fossil fuels causes acid rain.
C.
Canada is a land rich in mineral resources.
D.
The United States is rich in resources.

People have often acted as though these resources were unlimited. In recent years, however, the people of the United States have started to understand that they must use their resources more wisely than in the past.