Biomass is the amount of living matter in a given area, representing the stored chemical energy in the tissues of organisms. This stored energy is what can be passed on to the next trophic level when organisms are eaten, though only a portion makes it through due to metabolic costs and heat loss. Biomass is often measured as dry mass per area (like grams per square meter) or as carbon content, and it reflects a stock—an amount stored at a given time—not a rate of energy flow. The other options describe energy input from sunlight, the rate at which energy moves through the ecosystem, or the number of organisms, none of which capture the stored tissue energy available for consumption.

Understanding levels of biological organization helps explain this. An organ system is a group of organs working together to perform a function, and an organism is a living thing. Describing a single organ doesn’t define an organ system or an organism—it’s just one component, not the whole system or the whole living being. Describing a living cell isn’t a definition of either organ system or organism either, because it points to the cell level, the basic unit of life, which can be part of a larger organism but does not by itself define the entire system or the whole organism. So the statement about a living cell is the one that does not directly define an organ system or an organism.

An organ system is a group of organs that work together to perform a specific function for the body. Each organ brings its specialized role, and they coordinate—often through signals like hormones or nerves—to accomplish a larger task, such as digestion or circulation. For instance, the digestive system includes multiple organs like the stomach, intestines, liver, and pancreas, all collaborating to break down food, absorb nutrients, and remove waste. A single organ, like the stomach, handles only a part of that process. A cell is far smaller and not an assembled set of organs, and a living thing refers to the whole organism that contains one or more organ systems. Hence, the option describing a group of organs working together to perform a function best matches the concept.

An organ system is a group of organs working together to perform a function. This combined action is what enables the body to carry out complex tasks essential for survival, not just a single organ in isolation. For example, the digestive system includes several organs like the stomach, intestines, liver, and pancreas that coordinate to break down food, absorb nutrients, and manage waste. The heart and blood vessels participate in the circulatory system, coordinating to transport oxygen and nutrients throughout the body. The other statements don’t fit because a living thing is the entire organism, not just a system; a single organ isn’t an organ system; and a cell is far smaller and not an assembly of multiple organs.

Selective pressure is any factor in the environment that causes individuals with certain traits to have a higher chance of surviving and reproducing than others. This differential success changes how common those traits are over generations, driving evolution. It’s not a mutation itself, and it doesn’t mean the environment has no effect or that everyone is equally affected. Instead, it acts on existing variation, favoring traits that confer a survival or reproductive advantage. For example, predators may more often catch less camouflaged prey, so better-camouflaged individuals are more likely to survive and pass on their genes. Antibiotics can favor bacteria that carry resistance genes, increasing their frequency. Therefore, a selective pressure is best described as something that gives certain traits an advantage, shaping the population over time.