Which of the following is an example of a renewable natural resource?

a. the knowledge possessed by scientists
b. carpenters’ labor services
c. lumber
d. all of the above are correct.

c. lumber      

Explanation:

Lumber is considered a renewable natural resource because it is gotten from trees and trees are grow-able. A renewable natural resource is a resource which can be used repeatedly and replaced naturally. Examples of renewable natural resources are solar energy from the sun, water, oxygen, biomass, trees etc. Trees can be harvested and processed into lumber. The harvested trees can be planted again by humans or they can naturally reproduce through seedlings or fruits that drop and germinate on their own. This renewable germinating process of trees makes lumber a renewable natural resource.

there are no contact forces mentioned on the list of choices that you've provided.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.