单选题Directions: There are ten blanks in the
following passage. For each numbered blank, there are four choices marked A, B,
C and D. Choose the best one and mark your answer on the ANSWER SHEET with a
single line through the center. Fueled by weather, wind
and dry undergrowth, uncontrolled wildfires can burn acres of land—and consume
everything in their way—in mere minutes. {{U}} {{U}}
1 {{/U}} {{/U}},more than 100 000 wildfires clear 4 million to 5 million
acres of land in the U. S. every year. A wildfire moves at speeds of up to 23
kilometers an hour, consuming everything—tress, bushes, homes, even humans--in
its {{U}} {{U}} 2 {{/U}} {{/U}}. There are
three conditions that need to be {{U}} {{U}} 3 {{/U}} {{/U}}in
order for a wildfire to burn: fuel, oxygen, and a heat source. Fuel is any
material {{U}} {{U}} 4 {{/U}} {{/U}}a fire that will burn
quickly and easily, including trees, grasses, hushes, even homes. Air supplies
the oxygen a fire {{U}} {{U}} 5 {{/U}} {{/U}}to burn. Heat
sources help spark the wildfire and bring fuel to {{U}}
{{U}} 6 {{/U}} {{/U}}hot enough to start burning. Lightning, burning
campfires or cigarettes, hot winds, and even the sun can all provide {{U}}
{{U}} 7 {{/U}} {{/U}}heat to spark a wildfire.
{{U}} {{U}} 8 {{/U}} {{/U}}ofter harmful and destructive to
humans, naturally occurring wildfires play a positive role in nature. They
{{U}} {{U}} 9 {{/U}} {{/U}}nutrients to the soil by burning dead
or decaying matter. They remove diseased plants and harmful insects from a
forest ecosystem(生态系统). And by burning {{U}} {{U}} 10 {{/U}}
{{/U}}thick tress and bushes, wildfires allow sunlight to reach the forest floor,
enabling a new generation of young plants to grow.