4. defensive

B. 請用下面的文章出五題閱讀測驗題（無須抄題或改寫）。每題需有四個選項，並請附上正確答案。 Deserts and steppes are harsh environments where only very specialized plants and animals are capable of survival.While temperatures can soar up to 40-45 degrees Celsius during the day, they can also drop below freezing at night because there is no water vapor in the atmosphere to hold in heat. Rainfall is less than 25cm per year if it rains at all. This impossible situation has caused the cacti, the resident tree populations to evolve some very interesting attributes to the point that people usually don’t even look upon them as being trees. Most cacti live in habitats subject to at least some drought. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth. Cacti show many adaptations to conserve water. Every part of the organism has changed. Almost all cacti are succulents, meaning they have thickened and fleshy parts acclimatized to store water. Unlike many other succulents, thestem is the only part of most cacti where this vital process takes place. The root systems are usually exceptionally wide, shallow and highly saline to allow the most water absorption in the least amount of time. A full-grown Saguaro cactus, for instance, can imbibe 3,000 liters of water in 10 days. Since the tremendous, broad and flat leaves have become thin and sharply pointed, protective spines, the trunk and branches have developed the ability to perform photosynthesis. It is also covered with a wax that seals in moisture and normally has long, deep creases or ribs running along most or all of the length. These creases make it possible for a great amount of swelling to occur when rains do come. In this way, sufficient water can be accumulated to keep it alive for years without rain. What cannot be seen with the naked eye is the plant’s breathing and chemistry. Cacti do the equivalent of holding their breath during daylight hours. The pores that allow transpiration, called stomata, close when the sun is out to prevent excessive water loss. At night, the pores open; thus, oxygen, carbon dioxide and water vapor are allowed to pass through. Due to the fact that photosynthesis cannot happen in the dark, the carbon dioxide is piled up as malic acid, which is commonly found in unripe fruit. The next day, when the sun rises in the east, the stomata shut again, and the cactusperforms photosynthesis by using the cumulated carbon dioxide. If one degusted the pulp from a cactus at sunrise, it would taste sour. If one savored it at sunset, one would relish it owing to its sugary elements. In addition to all the features mentioned above, the general shape of the cactus is also an advantage. It has very few branches, and all structures tend toward cylindrical and spherical shapes, which maximize the ratio of volume to surface area for the purpose of protecting water reserves more easily. With so many unique adaptations, the cactus survives so well that it has become the most recognizable symbol of desert life.

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4. defensive

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