单选题 Lichens may grow on the bark of a tree in a steaming tropical rain forest, on the bricks of big city buildings, on rocks in hot springs, on wind-swept mountaintops, and in the driest desserts. In the arctic, they provide the principal food for caribou, and they are one of the few plants that grow in Antarctica. They are pioneers, appearing in barren rocky areas and starting the formation of soil in which mosses, then ferns, and then other plants can take root.
Lichens are a partnership of two plants — fungi and algae. The lichen body is made up of a network of fungal strands. In the upper layers of these grow groups of algae. The two organisms live together to the benefit of both, a relationship known as symbiosis. The fungi provide support, absorb water, and shelter the tender algae from direct sunlight. The algae carry on photosynthesis and provide the fungi with food. The algae can live independently and are recognizable as a species that grows alone. The fungi, on the other hand, cannot live apart from their partners. They can be placed in known classes of fungi but are unlike any species that lives independently.
So definite are the form, color, and characteristics of these double organisms that for hundreds of years, they were classified as one. More than 15,000 "species" were named. If these organisms are classified as separate species, it is difficult to fit them into the existing system of classification. But if they are classified separately, these species of fungi seem rather strange. Lichens are a splendid example of the difficulties faced by taxonomists in classifying species.

单选题 Lichens may NOT grow ______ .
A．in the tropical woods B．on the ceilings of city buildings
C．on rocks in hot springs D．in dry desserts
【正确答案】 B
【答案解析】[解析] 细节题型
见第一段第一句，可知选项A、C、D都被提及，所以B为答案。
单选题 What does the author imply about lichens in Paragraph 1?
A．They require a lot of moisture to live.
B．They primarily live in cold places.
C．They can live anywhere except around people.
D．They have adapted to a wide variety of environments.
【正确答案】 D
【答案解析】[解析] 推断题型
从第一段第一句可推出在多种气候、自然条件、环境下，Lichens均可生长，因此可知 D(Lichens可适应多种环境)为答案。
单选题 Why does the author call lichens pioneers?
A．Because they developed so early in the history of the planet.
B．Because of their primitive structure.
C．Because they prepare soil for other plants.
D．Because they were the first plants to live in Antarctica .
【正确答案】 C
【答案解析】[解析] 细节题型
见第一段最后一句的伴随状语：appearing in barren rocky areas and starting the formation of soil in which mosses，then ferns，and then other plants can take root(它们出现在荒芜的多岩石地区，使得土壤开始生成，然后各种植物才得以据此生根)可知C为答案。
单选题 The word "barren" underlined in Paragraph 1 most probably means _______ .
A．lifeless B．frigid C．hard D．uncovered
【正确答案】 A
【答案解析】[解析] 词义题型
单选题 Which of the following is an example of symbiosis as it is described in Paragraph 2 ?
A．Certain types of tall grass conceal tigers because of the tigers' striped markings.
B．Fish called remoras attach themselves to sharks and eat the scraps of the sharks' meals.
C．Mistletoe, a type of shrub, grows on trees and harms them by extracting water and nutrients.
D．Protozoa in the intestines of termites digest the cellulose that the termites eat, and their waste products nourish the termites.
【正确答案】 D
【答案解析】[解析] 细节题型
见第二段第四句The two organisms live together to the benefit of both，a relationship known as symbiosis．(这两种有机体为了双方的利益共同生存，它们间的关系被称之为“共生现象”)及随后的举例；而本题的四个选项中只有选项D(白蚁肠中的原生动物将白蚁吃下的纤维素消化，而它们的排泄物又为白蚁提供滋养)体现了这种共生现象，因此 D为答案。此外还可根据四个选项中的动词来判断是否有互利互存的关系来寻找答案。
单选题 Which of the following can be inferred about the effect of direct sunlight on lichens?
A．It damages the algae.
B．It helps the fungi absorb water.
C．It is required for the algae to carry on photosynthesis.
D．It destroys the fungi.
【正确答案】 A
【答案解析】[解析] 推断题型
见第二段第五句：The fungi为algae提供支持，为它吸收水分，并为稚嫩的algae免受阳光直射提供遮护；由此推断出直射的阳光会损害algae；因此A为答案。
单选题 According to the passage, the algae ______ .
A．may not carry on photosynthesis alone B．supply food to the fungi
C．can't live separately D．aren't recognizable as a species
【正确答案】 B
【答案解析】[解析] 细节题型
见第二段第六句：The algae进行光合作用并为the fungi提供食物。因此B为答案。
单选题 Why does the author say that "these species of fungi seem rather strange"?
A．They are larger than typical fungi.
B．Unlike other fungi, they can produce their own food.
C．They exist only as partners of algae.
D．They do not fit into any known class of fungi.
【正确答案】 C
【答案解析】[解析] 细节题型
见第二段倒数第二句、第三句：(The algae能独立生存且是公认的能独自生长的物种；而另一方面，The fungi没有同伴却无法生存)及第三段第三句：但是如果将它们分别地分类，这些fungi物种就显得格外奇怪了(因为它们是依赖同伴生存的)；因此C为答案。
单选题 Which of the following best expresses the main idea of Paragraph 2 ?
A．Because of their characteristics as double organisms, it is difficult to classify lichens.
B．Over 15,000 varieties of lichens have been identified.
C．Double organisms should always be classified as separate species.
D．Taxonomists always find it difficult to classify new species of plants.
【正确答案】 A
【答案解析】[解析] 主旨题型
见第二段第一句：Lichens是两种植物——fungi和algae的共同体，随后本段谈及两者间的互利共生的关系，其中一种无法独立生存，因此如将之分类将会十分困难；因此答案为A。
单选题 The word "splendid" underlined in Paragraph 3 most probably means ______ .
A．unique B．improbable C．excellent D．famous
【正确答案】 C
【答案解析】[解析] 词义题型
根据句意，选项C最符合原意；因此C为答案。