شرکت در آزمون آنلاین زبان انگلیسی 1 - زبان انگلیسی 1 | آزمون شماره 13042

The travelers must ..................... the river and come out on the other side if they want to keep moving.

Most animals use more than one species as food. Therefore, the term “food web” is a better description of the food relationship than the term “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 meat and rabbits, and 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 the aquatic chains is algae. Most algae are microscopic green plants that produce food by photosynthesis, a process in which energy from sunlight converts carbon dioxide and water to sugar. Tiny fish in lakes, streams, and oceans eat algae. In turn, they are eaten by larger fish. These larger fish are eaten by still larger fish. Algae make the food supply for fish. 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 producers, decomposers, and consumers. Organisms containing chlorophyll are producers. Thus, green plants are producers. Animals that eat other animals and plants are consumers. Microbes, one-celled organisms that cause the decay of the dead animals and plants, are decomposers. Since decomposers cannot make their food, they are also consumers.

 What is the main purpose of this passage?