I'm not positive but I think it means that the resources that have to be used are difficult to get a hold of or very rare. This could mean a few different things though.
The main economic resource of difficulty is scarcity of resources.
a) What is meant by the term "sustainable natural resources"?
forest
Natural Resource is a gift of nature to man kind used to produce goods and services.
Any resource may eventually be destroyed
Any resource may eventually be destroyed
Any resource may eventually be destroyed
the inability of competing firms to obtain resources from other firms
I think you meant DD points. Those are for diving. They stand for Degree of Difficulty.
. Fossil fuels(such as coal, petroleum, and natural gas
the difficulty of setting railroad schedules over long distances
animals were meant to be hunted