The problem referred to in “solutions to this problem” (l. 14-15) is: A) using old machinery; B) cleaning the environment; C) opening new digging ...

The problem referred to in “solutions to this problem” (l. 14-15) is:

A) using old machinery;
B) cleaning the environment;
C) opening new digging sites;
D) reclaiming damaged areas;
E) digging in unsuitable places