Which of the Following Best Explains the Falling Inflation-Adjusted Prices

Which of the following best explains the falling inflation-adjusted prices of most of the natural resources?

A) Most are renewable; therefore, the supply of natural resources is increasing.
B) Our ability to conserve natural resources is increasing faster than their depletion.
C) The demand for natural resources is diminishing due to the discovery of new substitutes.
D) New deposits of natural resources have been discovered.