What technology advancement was necessary before scientists could begin to observe cells

During convection, less dense fluid rises and denser fluid sinks.

Asap many people try to eliminate fat from their diets which is one reason it is necessary for humans to eat fat a) fat is the only way to get energy b) every cell in the body must contain fat c)fat stores less energy than polysaccharides d) saturated fats make blood vessels healthier

Polar zones are cold and tend to be dry because of a pressure system of cold. low high