Which statement best describes the variables in a controlled experiment?
a. the independent variable is changed to see its effect on the dependent variable while all other conditions are unchanged.
b. the dependent variable is changed to see its effect on the independent variable while all other conditions are unchanged.
c. the independent variable is allowed to change, and the dependent variable is held constant to see what other conditions show a relationship.
d. the dependent variable is allowed to change, and the independent variable is held constant to see what other conditions show a relationship.

The correct answer is A. The independent variable is changed to see its effect on the dependent variable while all other conditions are unchanged.

Explanation:

In most experiments, scientists study a phenomenon based on two variables or factors, these are the dependent and independent variables. In the case of the independent variable, this variable is purposefully manipulated by researchers. This is done to see if this variable affects the dependent variable which is the main factor being studied. Additionally, other conditions or variables are controlled to be constant or unchanged.

For example if you want to test if the quality of water affects the growth of plants, the independent variable or the factor you manipulate is the quality or type of water, the dependent variable is the growth of plants as this is the factor that you study and other conditions such as type of plant, light, soil, are the same for all plants. According to this, the statement that describes variables in a controlled experiment is "The independent variable is changed to see its effect on the dependent variable while all other conditions are unchanged".