<P> Resources change over time with technology and economics ; more efficient recovery leads to a drop in the ore grade needed . The average grade of the copper ore processed has dropped from 4.0 % copper in 1900 to 1.63 % in 1920 , 1.20 % in 1940 , 0.73 % in 1960 , 0.47 % in 1980 , and 0.44 % in 2000 . </P>
<P> Resources change over time with technology and economics ; more efficient recovery leads to a drop in the ore grade needed . The average grade of the copper ore processed has dropped from 4.0 % copper in 1900 to 1.63 % in 1920 , 1.20 % in 1940 , 0.73 % in 1960 , 0.47 % in 1980 , and 0.44 % in 2000 . </P>
what might change the resources that an economy has available over time
what might change the resources that an economy has available over time
technology
technology