The enemies of the forest are thus numerous and various and its proper protection depends on:

A. Knowledge of the various forms of remedying such damage
B. Knowledge of suitable methods of preventing or remedying such damage
C. The application of such knowledge to particular cases of damage, with due regard to the cost of the proposed protective operations
D. All of the above

The correct answer to the question: "The enemies of the forest are thus numerous and various and its proper protection depends on:" is "All of the above".