A、It is to determine which food chain is the most efficient B、It is to describe the food network among plants and animals. C、It

Most animals use more than one species as food. Therefore, the term "food web" is better description of food relationships than "food chain". A food web is a complex feeding system that contains several food chains. For example, mice, rabbits, and deer eat plants. Owls eat mice and rabbits. Mountain lions eat rabbits and deer. These five species are parts of food chains that together form a food web.
    The first link in a food chain is always a green plant. Only organisms with chlorophyll, such as green plants, can make food. For example, the first link in aquatic food chains is algae. Most algae are microscopic green plants that produce food by photosynthesis. In photosynthesis, energy from sunlight converts carbon dioxide and water to sugar. Tiny fish in lakes, streams, and oceans eat; in turn, these tiny fish are eaten by larger fish. The larger fish are eaten by still larger fish. The food supply for fish is made by algae. This food is then passed through the food chains as one animal eats another.
    Organisms may be divided into three groups based on how they obtain food. These groups are producer, decomposer, and consumer. Organisms containing chlorophyll are producers. Thus, green plants are producers. Animals that eat other animals and plants are consumers. Microbes, on-celled organisms that cause the decay of dead plants and animals are decomposers. Since decomposers cannot make their own food, they are also consumers.