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题目材料：
Some scientists speculate that projected increases in atmospheric carbon dioxide levels could increase crop yields and thereby help meet increased demand for food as the world's population rises. Most plant species are classified as either type C3 or C4, each type referring to a distinct photosynthetic pathway. C3 plants (over 90 percent of all plant species) benefit much more from carbo dioxide increases than do C4 plants: experiments suggest that doubled carbon dioxide levels can increase C3 growth rates, and hence C3 crop yields, by 20 to 35 percent, whereas C4 plants——including some important food crops such as maize and sugarcane and many pasture grasses on which livestock herds depend——benefit little. Most information regarding crop response to carbon dioxide comes from experiments in which yield-reducing competition from weeds was not a factor. Site-specific mixes of weed and crop species, crop species' responses to other environmental conditions such as precipitation and temperature, and likely changes in geographic ranges of insect, fungal, and bacterial crop pests as a result of carbon dioxide-related climate change——all these must be considered in projections of crop yields in the future carbon dioxide-rich world.
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