3. According to the fifth paragraph， under what conditions are algae able to use solar energy to make hydrogen?
A) When there is a lot of oxygen in the air.)
B) When there is no oxygen in the air.
C) When photosynthesis is taking place.
D) When enough starch is stored.
4. Researchers have met with difficulties when trying to make algae produce hydrogen efficiently. Which one of the following is one such difficulty?
A) It is not possible to remove sulfate from the environment.
B) It is not possible to work in an airfree environment to produce hydrogen.
C) It is not easy to make sugars instead of hydrogen.
D) It is too slow for algae to produce hydrogen when the sulfate is removed.
5. What is NOT true of algae?
A) They are easy to grow.
B) They can be a very good fuel source.
C) They are cheap to eat.
D) They can be used in many ways.
6. Why do some scientists study how plants convert sunlight carbon dioxide and water into sugars and starches?
A) Because they want algae to produce sugars and starch.
B) Because they want green plants to become a new source of energy.
C) Because they want to turn plant sugars to a new form of energy.
D) Because they want to make photosynthesis more efficient.
答案与题解：
1.C 根据文章第一段最后一句的意思，c是正确选择：植物从来就利用阳光作为能源。
2.B文章的第三段说，科学家企图将植物或植物类生物细胞作为微型光合能源供应站来研究，并举例说，他们正在用绿藻进行实验，若成功，绿藻所产生的氢将可用来为汽车的燃料电池充电。所以答案是B.
3.B 文章第四段告诉我们：…algae will produce hydrogen in an air free environment.没有空气的情况肯定就没有氧气了。该段最后一句又说，是氧阻止绿藻制造氢。
4.D选项A、B都不是正确答案，因为短文的第六段告诉我们，remove sulfate和work in an air free environment都是可能的，但问题是两者都有弊端，使氢的制造不那么容易。D所述内容就是困难之一：绿藻细胞因为没有了sulfate而减缓工作速度，因此产生不了多少氢。
5.C 根据短文最后一段的描述，绿藻可以在任何地方生长，它们是很容易使用的燃料能源，而且用途广泛，所以，A、B、C都是正确的描述，不是答案。C是答案，因为“The organisms are cheap to get and to feed”中的feed是“养殖”的意思，不能解释为“吃”。