Which of the following is a way that trees have been negatively impacted by human use?
a. deforestation
b. pollution
c. reduction
d. all of the above

All of the mentioned human actions have negative impact of the trees. The deforestation or the removal of the forests when trees are removed and destroyed. In such a situation the trees which left may not be able to survive the change. Tress are intolerant to air pollution. The pollution weakens them and limits their nutrition. Reduction , the chemical reaction resulsting in reduced oxygen also negative impact on the trees, because oxygen is needed in their respiration process.

Independent variable is something that is changed in a experiment.