2. 外语
  3. Coral reefs are one of the most fragile, biologically complex, and diverse marine ecosystem on Earth. This ecosystem is one of t

Coral reefs are one of the most fragile, biologically complex, and diverse marine ecosystem on Earth. This ecosystem is one of t

admin2009-04-18  101
问题 Coral reefs are one of the most fragile, biologically
complex, and diverse marine ecosystem on Earth. This
ecosystem is one of the fascinating paradoxes of the bio-
sphere: how do clear, and thus nutrient-poor, waters sup-
(5) port such prolific and productive communities? Part of the
answer lies within the tissues of the corals themselves.
Symbiotic cells of algae known as zooxanthellae carry out
photosynthesis using the metabolic wastes of the coral
thereby producing food for themselves, for their corals,
(10) hosts, and even for other members of the reef community.
This symbiotic process allows organisms in the reef com-
munity to use sparse nutrient resources efficiently.
Unfortunately for coral reefs, however, a variety of
human activities are causing worldwide degradation of
(15) shallow marine habitats by adding nutrients to the (water.
Agriculture, slash-and-burn land clearing, sewage disposal
and manufacturing that creates waste by-products all
increase nutrient loads in these waters. Typical symptoms
of reef decline are destabilized herbivore populations and
(20)an increasing abundance of algae and filter-feeding
animals.
Declines in reef communities are consistent with observa-
tions that nutrient input is increasing in direct proportion to
growing human populations, thereby threatening reef com-
(25) munities sensitive to subtle changes in nutrient input to
their waters.
选项 A、describing the effects of human activities on algae in coral reefs
B、explaining how human activities are posing a threat to coral reef communities
C、discussing the process by which coral reefs deteriorate in nutrient-poor waters
D、explaining how coral reefs produce food for themselves
E、describing the abundance of algae and filter-feeding animals in coral reef areas
答案B
解析 This question asks you to identify the primary concerns of the passage. The best answer is B. The first paragraph describes how coral reefs survive in nutrient-poor waters. The second paragraph states that human activities are adding nutrients to shallow marine habitats, thus threatening the survival of coral reef communities. Choices A and E can be eliminated because an increase in algae and filter-feeding animals is mentioned in the context of the passage’s larger concern with the negative effects of human activities on coral reefs. Choice C is incorrect because the first paragraph of the passage explains how coral reefs survive in nutrient-poor waters; the deterioration of coral reef communities has occurred as a result of the increase in nutrients in their habitats. Choice D can be eliminated because the first paragraph states that symbiotic cells of algae known as zooxanethellae produce food for coral reef communities.
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