The amount of food, water, and air might influence the population. If there is not enough of any of these, the population will decrease. This is because those factors are needed to survive.
Although the actual questions are missing from your ask, here's how you can tell.
If the study will be messed up (added bias) through interaction with the subjects the study should be observational. For example if a biologists wanted to study the eating habits of wild birds in the tropics, they would need to do an observational study. if they did a controlled experiment, the birds will change their eating habits based on human interaction, thus making the study invalid.
If the study requires specific measurements, close monitoring, or contact and interaction with the subjects the study should be a controlled experiment. For example, if a scientist wanted to test out a new drug, the health of patients would need to be closely monitored.
Answer:
The Liver
Explanation:
The liver's main job is to filter the blood coming from the digestive tract, before passing it to the rest of the body. The liver also detoxifies chemicals and metabolizes drugs. As it does so, the liver secretes bile that ends up back in the intestines.
Bacteria protects the cell in your intestine and helps us absorb fatty acids. It is harmful because when you have contact with it, the bacteria can reproduce in your body, damaging it and making you sick.
