If fish and frogs decrease in a river, which understanding of the food chain is necessary?

Correct answer: Small organisms can be food base for larger organisms

Direct answer: Option A is correct. A food chain shows how food and energy pass from one organism to another. In a river, very small organisms such as algae, plankton, insects, or other tiny water animals may be eaten by larger animals. Fish and frogs may then be eaten by snakes, birds, or other predators. Therefore, if fish and frogs decrease, the food relationships of the whole river ecosystem may be disturbed. Option A is correct because small organisms can form the food base for larger organisms. Option B is wrong because small organisms are important links and their loss can affect many animals. Option C is wrong because aquatic animals must eat food to obtain energy. Option D is wrong because a river is itself an aquatic habitat and strongly affects the organisms living there. Memory cue: every level of a food chain matters; removing a small link can affect larger links.