题目内容：

    The past 70 years have been good for corn production in the Midwestern United States, with yields increasing fivefold since the 1940s. Much of this improvement has been credited to advances in farming technology but researchers at Harvard University are asking if changes in climate and local temperature may be playing a bigger role than previously thought.

In a newspaper, researchers find that a prolonged(延长的) growing season due to increased temperatures, combined with the natural cooling effects of large fields of plants, have had a major contribution to improved corn production in the U.S. “Our research shows that improvements in crop yields depend, in part, on improvements in climate,” said Peter Huybers, professor of Earth and Planetary Sciences. “In this case, changing temperatures have had a beneficial impact on agricultural production, but there is no guarantee that benefit will last as the climate continues to change.

Understanding the detailed relationships between climate and crop yields is important as we move towards feeding a growing population on a changing planet. ” The researchers modeled the relationship between temperature and crop yields from 1981 to 2017 across the so-called Corn Belt. They found that as temperatures increased due to global climate change, planting days got earlier and earlier, shifting by about three days per decade. Early planting means the corn has more time to mature before the end of the growing season.

There is also a second, more surprising trend that has benefited corn yields. Whereas the vast majority of temperatures have warmed over the last century, the hottest days during the Midwestern growing season have actually cooled.

The researchers estimate that more than one quarter of the increase in crop yields since 1981 can be attributed to the twin effects of a longer growing season and less exposure to high temperatures, suggesting that the crop yield is more vulnerable to climate change than previously thought.

The researchers also show that the planting and harvest dates farmers currently use is significantly better adapted to the present climate than it would be to climates in earlier decades.

1.What is previously thought to be the reason for the increase of corn production?

A.Local temperature change. B.Advanced farming technology.

C.Climate change. D.Production efficiency.

2.Why is it important to understand the relationships between climate and crop yield?

A.The climate has a beneficial impact on agriculture.

B.We can change the climate to improve crop yield.

C.We depend on the climate to get food.

D.Crop yield relies less on the climate.

3.How many factors that benefit corn yields are mentioned in the text?

A.4. B.3.

C.2. D.1.

4.What is the main purpose of the text?

A.To explain a mystery. B.To question a common sense.

C.To show a new trend. D.To introduce a research result.