Our Earth Has Many Resources,but They Are Not Unlimited

Multiple Choice

Our Earth has many resources,but they are not unlimited.In the case of abundant metals,supply will most often meet demand.However,problems can occur when

A) there is a shortage of ores that can be mined at a reasonable cost.
B) high grade ores are depleted and low-grade ores need to be mined.
C) transport and processing costs override economic feasibility.
D) all of the above.