D is the right for the question I guess and I got it right
<span>Science is a practice of gaining comprehension of the nature of materials and phenomena by gathering and evaluating evidence. A scientific study is based on an hypothesis, a theory of causality. As the body of scientific knowledge increases, the design of scientific studies is informed by previous discoveries.</span>
The dependant variable would be the salmon, because they depend on the water and pollution levels to give a result.
Alveolar walls are thin to enable the quick passage of oxygen in and out of the lungs. The lungs works hand-in-hand with the heart and also is responsible in converting blood that's de-oxygenated into an oxygenated blood that is usable by the organ systems of the body. The alveolar walls are also designed to have thin membranes so that it can expand and constrict quickly allowing us to breather properly.