下列关于生态系统的描述，正确的一项是（    ）

阅读理解

How can we reduce the risk?

    There are four general approaches to dealing with volcanic dangers. We can try to keep the danger from occurring - often an impossible task. We can try to change its path or reduce its impact on existing development. We can take steps to protect future development. We can also do our best to have disaster response plans in place before they are needed.

Removing the Threat

    Clearly, there is no way to stop an eruption. We can, however, attempt to reduce the eruption's effects by strengthening structures, for example, building protective works such as walls to make lava(熔岩) flow away from developed areas. Such efforts can be and have been successful, but are of limited use in a large-scale eruption.

Planning for the Future

    Protecting future development from volcanic dangers is a simple task. Before building houses, we should judge the risk. If the risk seems too great, a safer location should be found. This type of planning is very effective, but all too often, people are drawn to the lush(葱郁的)，rolling land of a quiet volcano.

Disaster Preparedness

    When a volcano comes to life, a few weeks may not be enough time to avoid a tragedy. Planning is the key to saving lives. Well before the warning signs occur, people must be educated about volcanic dangers. Escape plans must be in place. Communication between scientists, officials, the media, and the general public should be practiced. Emergency measures must be thought out and agreed upon.

    If you doubt the importance of these efforts, take another look at past volcanic tragedies, such as the eruption of Nevado del Ruiz. Communication failures left the town of Armero unprepared for escape. When a deadly mudflow came down the slope(斜坡), 21,000 people— 90 percent of the town's people--died.