When has a mineral resource been economically depleted? 

A)  When it costs more to mine the mineral than it is worth. 
B)  When 50% of the minerals have been mined. 
C)  When 80% of the minerals have been mined. 
D)  When there is no mineral left to mine.

Economically Depleted

Refers to a condition where a resource has been exploited to the point where it is no longer profitable to extract or use.

Mineral Resource

Natural concentrations of minerals or, in some cases, organic material, that can be extracted and processed into useful products at a profit.