In the experiment, the independent variable is the drug amount. Experiment group is given certain amount of drug, control group is not given the drug. The dependent variable is the body fat. I think you means the control is the living environment including food and water.
I would say that Melissa has made the mistake of using the same uncleaned cutting board first for cutting up chicken which is notorious for forming bacteria quickly when out of the fridge and then chopping vegetables on it thus running the possibility of contaminating the vegetables with chicken bacteria. She should ensure she washes the cutting board carefully with soap and water to avoid contamination of the vegetables.
A decrease in dissolved oxygen.
This is because oxygen is a byproduct of photosynthesis. If the plants are in water, some of the oxygen they produce will become dissolved in it. Without those plants, you have less dissolved oxygen.
Its C i promise you i just toke the quiz and got an 100 on it