Base your answer on the information below and on your knowledge of biology. euglena are single-celled organisms that live in ponds. all euglena have chloroplasts and can make their own food. they can also take in food from the environment. the diagram below represents a euglena. an experiment was set up to determine the effect of nitrates, a pollutant, on the number of chloroplasts present in euglena. five tanks were set up, each with euglena and a different concentration of nitrate solution: 0%, 0.5%, 1.0%, 1.5%, and 2.0%. the tanks were placed in a sunny location where each tank received the same amount of light. which statement correctly identifies a variable in this experiment?
a. the independent variable is the concentration of nitrate solution used.
b. the dependent variable is the number of euglena placed in the tanks.
c. the independent variable is the amount of sunlight.
d. the dependent variable is the number of tanks used.