you have selected the following prediction to test: Previously thinned forests will have higher tree survival than adjacent forests that were not thinned. Now you need to define the key variables that you will use to test this prediction. For the following variables, sort them as independent variables, dependent variables, or variables that should be controlled (held constant)1. thinning or no thinning before the fire elevation of the site 2. dominant tree species 3. tree survival rate after the fire A. Independent variableB. Dependent variable C. Controlled variable

A. Independent variable 2. dominant tree species

B. Dependent variable  3. tree survival rate after the fire

C. Controlled variable 1. thinning or no thinning before the fire elevation of the site

Explanation:  

A. An independent variable doesn't undergo changes during the experiment because it is not conditioned by other variables in the experiment. In this example, dominant tree species represent a variable that is independent of the experiment itself.

B. A dependent variable is those that you need to measure and depends on the other factors in the experiment. In this example, tree survival is dependent on the factors that affect the results (i.e. humidity in the trees, presence of resinous material in the trees that are non-combustible, etc.)

C. A controlled variable is those that we use to observe an expected outcome in the experiment and therefore it is used to compare with the test group. In this case, thinning or no thinning are used as positive and negative controls, respectively, because they enable their comparison with the test group in the experiment.  

correct answer is a. and d.