Any ornamental garden has both plants and structural elements such as rocks and fences. Because the plants constantly change with growth and decay, it is difficult to keep a proper balance in the garden between the plants and the structures. Balance can be achieved only with careful planning, and even when established, balance has to be constantly maintained by pruning and thinning. Which one of the following most accurately expresses the main conclusion of the argument? ['The reason why it is difficult to keep a proper balance in a garden between structures and plants is that the plants constantly change with growth and decay.', 'It is difficult to keep a proper balance in a garden between structures and plants.', 'Without careful planning and maintenance, the balance in a garden between structures and plants constantly changes with growth and decay.', 'It is difficult to constantly maintain the balance in a garden between structures and plants by pruning and thinning.']