The average temperature around the world is rising. People living in the US Midwest might find this fact hard to believe, though. Two new studies show that in America's heartland, summers are now cooler and wetter than they were in years past. The scientists suggest that the change in the Midwest climate may have happened because of farming.     The first study was led by David Changnon, a climatologist at Northern Illinois University in DeKalb. A climatologist studies the climate of an area, which includes measuring rainfall, temperature and wind. Climatologists want to know how these factors have changed in the past, and how they'll change in the future.     Changnon and his team studied temperature records from Chicago and 13 other sites in the Midwest. "These two changes--lower temperatures and more rainfall--may be connected by humidity," Changnon says. Humidity is the measure of how much moisture is in the air. Humid air, which contains a lot of moisture, takes longer to heat up than dry air.     So where did the extra  _ in the air come from? Changnon points to farms in the region. As plants grow, they pull moisture from the ground and release it into the air. And among plants, soybean and corn plants release a lot of moisture. Midwestern farms now plant more soybeans and corn than in the past, with 97 percent of farmland today planted with these two crops. He also notes that the plants are planted closer together now than they used to be, so there are more plants per acre than in the past.     The second study was led by famous scientists from the US. The researchers studied a vast area of the United States that stretches from South DaKota to Oklahoma and the Texas panhandle. They found that in 1930, farmers in that region irrigated only about 1.8 million acres of farmland, an amount roughly half the size of Connecticut. In 1980, however, farmers irrigated nearly 15 million acres--more land than Vermont and New Hampshire combined. According to the passage, which factors will you refer to if you want to study the climate of an area?
Sunlight and rainfall. Rainfall and longitude. Rainfall and altitude. Rainfall and wind.
Question: The average temperature around the world is rising. People living in the US Midwest might find this fact hard to believe, though. Two new studies show that in America's heartland, summers are now cooler and wetter than they were in years past. The scientists suggest that the change in the Midwest climate may have happened because of farming.     The first study was led by David Changnon, a climatologist at Northern Illinois University in DeKalb. A climatologist studies the climate of an area, which includes measuring rainfall, temperature and wind. Climatologists want to know how these factors have changed in the past, and how they'll change in the future.     Changnon and his team studied temperature records from Chicago and 13 other sites in the Midwest. "These two changes--lower temperatures and more rainfall--may be connected by humidity," Changnon says. Humidity is the measure of how much moisture is in the air. Humid air, which contains a lot of moisture, takes longer to heat up than dry air.     So where did the extra  _ in the air come from? Changnon points to farms in the region. As plants grow, they pull moisture from the ground and release it into the air. And among plants, soybean and corn plants release a lot of moisture. Midwestern farms now plant more soybeans and corn than in the past, with 97 percent of farmland today planted with these two crops. He also notes that the plants are planted closer together now than they used to be, so there are more plants per acre than in the past.     The second study was led by famous scientists from the US. The researchers studied a vast area of the United States that stretches from South DaKota to Oklahoma and the Texas panhandle. They found that in 1930, farmers in that region irrigated only about 1.8 million acres of farmland, an amount roughly half the size of Connecticut. In 1980, however, farmers irrigated nearly 15 million acres--more land than Vermont and New Hampshire combined. According to the passage, which factors will you refer to if you want to study the climate of an area?
A. Sunlight and rainfall.
B. Rainfall and longitude.
C. Rainfall and altitude.
D. Rainfall and wind.
Answer: