Starting in 1972, the National Park Service established a policy for forest fires called Natural Bum. It was true ____ some forest fires, such as those which were caused by lightning were necessary for forests____ (keep) balanced ecosystems, so the fire should be allowed to burn____ , a big fire in Yellowstone National Park in 1988 caused this policy to be abandoned (放弃) since the fire was initially allowed to burn yet soon out of control. As a result, the fire of 1988 destroyed much of Yellowstone, which is America's ____(old) and most beloved national park. Massive areas of plants were destroyed, and large empty spaces and acres of burned and blackened trees greeted ____ (visit). The rivers and streams were choked with ash, and the ecosystem of the park was changed ____ repair.

    In addition, great numbers of animals____ (kill) by the fires that burned out of control. The fires were driven by high winds____ (move) as many as ten miles a day. Many small animals died in the flames. The fires' rapid advances ____ (give) the wildlife little chance to escape. Even today, few of these small forest animals have returned to live in the park. In the years immediately following the fires, the numbers of visitors declined____ (rapid).