题目内容：
Questions are based on the following passage． You might think a little global warming is good for farming．Longer，warmner growing seasons and more carbon dioxide（CO2）--what plant wouldn’t love that? The agricultural industry basically agrees on that．But global warming’s effects on agriculture would actually be quite complicated--and mostly not for the better．
It’s true that some crops will prosper on a warmer planet，but the key word there is “some．”According to a government report，higher CO2 levels and longer growing seasons will increase yields for fruit growers in the Great Lakes region．But many major American crops already use CO2 so efficiently that more of it probably won’t make much difference to them．
What will make a difference are all the other things we’ll have more of as temperatures rise—namely droughts（干旱），bugs and big storms．More droughts mean lower crop yields．Melting snow in the Western U．S．will increase water availability in spring but decrease it in summer, forcing farmers to change cropping practices．As insects that eat crops adapt their migration patterns to our warmer climate，farmers will have to either use more insecticide or plant hardier crops．
Farmers on both coasts are already starting to reap some of what the nation’s fossil-fuel addiction has sown．Crops in those regions require a certain number of colder days，or ‘‘winter chill’’ before they break dormancy（冬眠）and begin flowering．Too few cold days breaks the plants’ flowering schedule which in turn affects pollination（授粉）and hurts yield．
So, given how much is at stake for them，how are farm states working to shape climate legislation? In response to agricultural demands，the Waxman-Markey bill frees the agricultural industry from CO2 emission limits and gives up control over what activities guarantee carbon offset credit to the Agricultural Department．
Some farmers--and some farm state congressional leaders--have argued that because plants convertC02 into oxygen，agricultural lands store more CO2 than they emit．This is only theoretically true．What we can say with certainty is that，like most big industries，farming is fossil-fuel intensive--large quantities of CO2 are emitted from farm equipment such as irrigation pumps and tractors．
Under the influence of global warming, the yields of American crops will __________． A．be greatly enhanced
B．be certainly reduced
C．still remain stable
D．be hard to predict
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