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  3. Researchers have made significant technological progress toward increasing the amount of plastic that plants can grow and

Researchers have made significant technological progress toward increasing the amount of plastic that plants can grow and

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问题             Researchers have made significant technological progress toward increasing the
       amount of plastic that plants can grow and altering the composition of the plastic to
       endow it with useful properties, but achieving both a useful
Line    composition and high plastic content in the plant proves to be a formidable task.
(5)     The chloroplasts of the leaves have so far shown themselves to be the best
       location for producing plastic, but the chloroplast is the green organelle that
       captures light, and high concentrations of plastic could thus inhibit
       photosynthesis and reduce grain yields, and the challenges of separating the
       plastic from the plant are awesome. Given sufficient time and funding,
(10)     researchers could overcome these technical obstacles, but a greater concern has
       made us question whether those solutions are worth pursuing. When calculating
       all the energy and raw materials required for each step of growing a
       biodegradable plastic made from plant sugar polyhydroxyalkanoate in plants, we
       discovered that this approach would consume even more fossil resources than
(15)     most petrochemical manufacturing routes.
           In a recent study, scientists found that making one kilogram of
       polyhydroxyalkanoate from genetically modified corn plants would require about
       300 percent more energy than the 29 megajoules needed to manufacture an
       equal amount of fossil fuel-based polyethylene (PE). Given this comparison, it
(20)     is impossible to argue that plastic grown in corn and extracted with energy from
       fossil fuels would conserve fossil resources. What is gained by substituting the
       renewable resource for the finite one is lost, through a tremendous irony, in
       the additional requirement for energy. Fueling this process requires 20 to 50
       percent fewer fossil resources than does making plastics from petroleum, but it
(25)     is still significantly more energy intensive than most petrochemical processes
       are, even though developing alternative plant-sugar sources that require less
       energy to process, such as wheat and beets, would eventually attenuate the use
       of fossil fuels.
           The energy necessary for producing plant-derived plastics gives rise to a
(30)     second, perhaps even greater, environmental concern: fossil petroleum is the
       primary resource for conventional plastic production, but making plastic from
       plants depends mainly on coal and natural gas used to power the corn-farming
       and corn-processing industries. Any of the plant-based methods, therefore,
       involves switching from a less abundant fuel (petroleum) to a more abundant
(35)     one (coal). Some experts argue that this switch is a step toward sustainability.
       Missing in this logic, however, is the fact that all fossil fuels used to make
       plastics from renewable raw materials (corn) must be burned to generate
       energy, whereas the petrochemical processes incorporate a significant portion
       of the fossil resource into the final product. Burning more fossil fuels
(40)     exacerbates an established global climate problem by increasing emissions of
       greenhouse gases, such as CO2, and SO2, which contribute to acid rain; thus
       the environmental benefit of growing plastic in plants is overshadowed by
       unjustifiable increases in energy consumption and gas emissions.
According to the passage, the new means of manufacturing plastic under consideration do not involve
选项 A、corn
B、plant sugar polyhydroxyalkanoate
C、petroleum
D、wheat
E、chloroplast plants
答案C
解析
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