A tall tree can transport a hundred gallons of water a day from its roots deep underground to the treetop. Is this movement propelled by pulling the water from above or pushing it from below? The pull mechanism has long been favored by most scientists. First proposed in the late 1800's, the theory relies on a property of water not commonly associated with fluids: its tensile strength. Instead of making a clean break, water evaporating from treetops tugs on the remaining water molecules, with that tug extending from molecule to molecule all the way down to the roots. The tree itself does not actually push or pull; all the energy for lifting water comes from the sun's evaporative power.
25. The passage is primarily concerned with
A. refuting a hypothesis advanced by scientists
B. discussing the importance of a phenomenon
C. presenting a possible explanation of a phenomenon
D. contrasting two schools of thought
E. discussing the origins of a theory
26. Consider each of the choices separately and select all that apply.
Which of the following statements is supported by the passage?
A. The pull theory is not universally accepted by scientists.
B. The pull theory depends on one of water's physical properties.
C. The pull theory originated earlier than did the push theory.
多选题型。正确选项AB。A项对应文章The pull mechanism has long been favored by most scientists，也就说明了not universally accepted by scientists。B项为文中细节：水的张力，改写符合一贯研究的解题经验：用抽象宏观的词替代原文具体内容。C项考察了关键的推理严谨性：尽管文中说长久以来pull theory被很多人接受，但没有明确提到任何两理论时间先后比较的内容。
27. The passage provides information on each of the following EXCEPT
A. when the pull theory originated
B. the amount of water a tall tree can transport
C. the significance of water's tensile strength in the pull theory
D. the role of the sun in the pull theory
E. the mechanism underlying water's tensile strength
属于传统上GRE阅读中比较难细节题，无法从题干本身获得定位主题词的信息。A项文中对应时间1800，B项所提最小单位可能是水分子。C项提到的tensile strength为理论涉及的基础性原理，26题已经有所涉及。D项对应文章末句。而E选项对文中信息做了推广延伸。在tensile strength更底层的具体原理机理文中没有进一步解释。
Producing and using biodiesel, a fuel derived from cultivated rapeseed, causes 35 percent less air pollution per gallon than does producing and using regular diesel fuel. The government plans to reduce diesel-related air pollution over the next decade by 25 percent, so replacing regular diesel with biodiesel would seem to be the obvious solution. Unfortunately, the greatest possible production of biodiesel would amount to only one percent of all diesel fuel to be produced during the next 15 years.
28. The passage is structured to lead to which of the following as a conclusion?
A. The use of biodiesel will increase by less than one percent over the next 15 years.
B. During the next 15 years, the production of biodiesel will be increased as fast as it is possible to increase it.
C. During the next 15 years, it will be impossible, just by switching to biodiesel, to meet the government's stated goal with respect to reducing air pollution.
D. Fifteen years from now, the air pollution caused by the production and use of one gallon of regular diesel fuel will be far less than it currently is.
E. There will be no significant year-to-year increase in the amount of regular diesel fuel used during the next 15 years.