In nature, plants and animals are perpetually engaged in mutualism because

Analyze the question: Mutualism is a relationship in which both organisms benefit. The question asks why plants and animals are always in a mutualistic relationship in nature.

Concept: Plants and animals exchange gases in a way that benefits both:

• Animals respire, taking in oxygen and releasing carbon dioxide.
• Plants photosynthesise, taking in carbon dioxide and releasing oxygen.
• Plants also respire, but in daylight photosynthesis far outweighs respiration.

So each group uses the other's waste gas, and both benefit.

Checking the options:

• A. they are rivals: Rivalry means competition, which is not mutualism. Wrong.
• B. all animals rely on food produced by plants: This is true for the most part, but it is one-sided. Animals benefit directly from plants' food, while plants do not get food from animals in return. It describes dependence, not mutualism. Wrong.
• C. they utilize respiratory wastes of each other: Animals' CO₂ is used by plants for photosynthesis, and plants' O₂ is used by animals for respiration. Both benefit, so this is mutualism. Correct.
• D. they are neighbours: Living close together does not mean a mutual benefit. Wrong.

Exam tip: In a mutualism question, look for the option where both sides gain. If an option describes only one party benefiting (like B), it is probably not the answer. Remember the gas-exchange cycle: animals give CO₂ to plants, and plants give O₂ to animals.