Answer:
1) The control /constant in this experiment is the fifth group of plants that are not exposed to any pollutants.
2) The dependent variable is the condition and growth of the plants.
3) The independent variable is the exposure of the plants to various pollutants.
4) The variables are quantitative because they can be measured.
5) Possible sources of unavoidable experimental error include the type of pollutants used, the amount of pollutants used, and the exposure time of the plants to the pollutants.
Further explanation of concepts:
What is a control variable?
A control variable is a variable that is kept constant in an experiment. Keeping the variables constant allows the experimenter to see the effects of the manipulated variable on the dependent variable. In order to control variables, the experimenter must use a controlled experiment. A controlled experiment is an experiment in which only one variable is changed while all other variables are kept constant.
What is a dependent variable?
A dependent variable is a variable that is being measured in an experiment. The dependent variable is the variable that is affected by the independent variable.
What is an independent variable?
The independent variable is the variable that is affecting the dependent variable. For example, in an experiment to test the effect of different fertilizers on plant growth, the independent variable would be the type of fertilizer used. The dependent variable would be the plant's growth.
Quantitative vs qualitative variables
Quantitative variables are variables that can be measured. Qualitative variables are variables that cannot be measured. Quantitative variables are usually numeric, while qualitative variables are usually non-numeric.