A single variable.
In an experiment we are trying to figure out how one thing (independent) effects another thing (dependent) so we want to keep everything constant. We want that experiment to be controlled so nothing else is effecting our dependent variable. That way we can be 100% sure that any change to our dependent is due to our independent variable. Hope that makes sense
That would be the circulatory system.... hint the word circulation.
Just like an assembly line, the human digestive system has different jobs that have to be done in order for the nutrients and such to be distributed properly throughout the body.
In any population, members have slight differences among them, whether in height, eyesight acuity, beak shape, rate of egg production, or other traits that may affect survival and reproduction. If a trait has a genetic basis, it can be passed on to offspring helping the terms of natural selection.