【答案解析】Since the cost of wind power electricity is comparatively high, British government introduced "tradable certificate" to bridge the gap. The wind power installations gain one certificate for each megawatt-hour they generate. Power distribution companies can buy the certificates as an alternative to paying the fines they would be levied for failing to buy a set proportion of renewable energy annually. But since the nearer the industry gets to meeting the government's requirements, the less the value of the certificates, the certificates, in fact, cap the supply of renewable energy instead of encouraging it. Firms will buy only the minimum amount of renewable energy necessary to comply with the law.