Paragraph 1: Renewable energy sources such as wind and solar do not always produce electricity at the moment people need it. That mismatch is one reason energy storage receives so much attention. Paragraph 2: Skeptics point out that batteries are expensive, require materials to manufacture, and can lose capacity over time. These concerns do not erase the need for storage, but they prevent easy optimism. Paragraph 3: The challenge is to develop storage systems whose costs and materials fit the scale of the energy transition. Storage is not a minor accessory; it is part of making renewable power dependable. Which description best explains paragraph 2's contribution to the full passage?