题目内容：

    Since 1970, North America has lost more than 2.9 billion birds, according to a study recently published in the journal Science. In less than half a century, the bird population has declined by some 29 percent, or more than 1 in 4 birds. For the first time, researchers have discovered that rare species aren't the only birds suffering population loss. In fact, common birds-including beloved backyard friends like sparrows and blackbirds-are taking the biggest hit.

The new study used nearly 50 years of data collected largely by bird watchers and citizen scientists. "A total of 419 native migratory species experienced a net loss of 2.5 billion individuals," the study says. More than 90 percent of the total loss can be found among just 12 bird families. Some of these widespread birds can thrive almost anywhere. The large-scale loss of these hardy birds reveals they're struggling to survive throughout the world/

Birds are considered indicator species because they're crucial to ecosystems, and their populations anticipate the environment's health. Birds are "amazingly efficient" at spreading seeds. Birds also keep insect populations in check. Even the smallest bird helps control the spread of plants or insects.

The new study, however, identifies the losses without knowing all the causes. Scientists do identify habitat loss as the biggest challenge. Climate change, disrupted migration patterns and pesticide use are also major factors. Industrial agriculture may present a bigger challenge. The species disappearing most rapidly—sparrows, blackbirds and larks—are associated with agriculture. In recent decades, agriculture has grown to produce more food, but cleared landscapes have become unlivable for birds.

“We need to change the way we grow food. Birds are signaling that we are doing it wrong, "as the study says. The study offers the example of the 1990s effort to switch to no-till farming, which reduced greenhouse gases and soil erosion（侵蚀）. “Farmers are innovators. And they've done it before. This is a great opportunity to have a huge impact on birds."

1.What's the study mainly about？

A.The climate issues. B.The disappearing birds.

C.The agricultural problems. D.The environmental protection.

2.How is the second paragraph mainly developed？

A.By listing figures. B.By giving examples

C.By analyzing reasons. D.By making comparisons.

3.What's the main reason behind the birds' habitat loss？

A.Climate change. B.Greenhouse gases.

C.Industrial agriculture. D.Disrupted migration patterns.

4.What does the study suggest as a solution？

A.Producing more food. B.Clearing more landscapes.

C.Reducing the use of pesticide. D.Adopting new means of farming.