单选题 Read the following text. Choose the best word or phrase marked A, B, C or D for each numbered blank.
Tropical rainforests are the most {{U}} {{U}} 1 {{/U}} {{/U}}ecosystem (生态系统) on Earth, and also the oldest. Today, tropical rainforests cover only 6 percent of the Earth's ground surface, but they arc home to over half of the planet's plant and animal {{U}} {{U}} 2 {{/U}} {{/U}}.
These forests receive between 160 and 400 inches of rain per year. The total {{U}} {{U}} 3 {{/U}} {{/U}}rainfall is spread pretty {{U}} {{U}} 4 {{/U}} {{/U}}throughout the year, and the temperature rarely dips below 60 degrees Fahrenheit. This {{U}} {{U}} 5 {{/U}} {{/U}}climate is due to the position of rainforests on the globe. Since rainforests are at the middle of the globe, located near the equator, they are not especially {{U}} {{U}} 6 {{/U}} {{/U}}by climate change. They receive nearly the {{U}} {{U}} 7 {{/U}} {{/U}}amount of sunlight, and therefore heat, all year. {{U}} {{U}} 8 {{/U}} {{/U}}, the weather in these regions remains fairly {{U}} {{U}} 9 {{/U}} {{/U}}
The consistently wet, warm weather and ample sunlight give plant life everything it needs to {{U}} {{U}} 10 {{/U}} {{/U}}. Trees have the resources to grow to {{U}} {{U}} 11 {{/U}} {{/U}}heights, and they live for hundreds, even thousands of years.
Rainforests are home to the {{U}} {{U}} 12 {{/U}} {{/U}}of animal species in the world. And a great number of species who now live in other environments, including humans, originally {{U}} {{U}} 13 {{/U}} {{/U}}the rainforests. Researchers {{U}} {{U}} 14 {{/U}} {{/U}}that in a large rainforest area, there may be more than 10 million different animal species.
In the past hundred years, humans have begun destroying rainforests at an alarming {{U}} {{U}} 15 {{/U}} {{/U}}. Today, roughly 1.5 acres of rainforest are destroyed every second. People are cutting down the rainforests in {{U}} {{U}} 16 {{/U}} {{/U}}of three major resources: 1. Land for crops. 2. Lumber for paper and other wood products. 3. Land for livestock pastures. In the current economy, people obviously have a need for all of these resources. But almost all experts agree that, over time, we will suffer much more {{U}} {{U}} 17 {{/U}} {{/U}}the destruction of the rainforests than we will benefit.
The world's rainforests are an extremely valuable natural resource, to be sure, but not for their lumber or their land. They are the main cradle of life on Earth, and they hold millions of {{U}} {{U}} 18 {{/U}} {{/U}}life forms that we have yet to discover. Destroying the rainforests is {{U}} {{U}} 19 {{/U}} {{/U}}to destroying an unknown planet—we have no idea what we're losing. If deforestation continues at its current rate, the world's tropical rainforests will be {{U}} {{U}} 20 {{/U}} {{/U}}out within 40 years.