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

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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 para- doxes of the biosphere: how do clear, and thus nutrient-poor waters support such prolific and productive communities? Part of the answer lies within the tissues of the corals themselves. Symbi- otic cells of algae known as zooxanthel-lae carry out photosynthesis using the metabolic wastes of the coral thereby producing food tor themselves, for their corals, hosts, and even for other mem- bers of the reef community. This symbiotic process allows organisms in the reef community to use sparse nutrient resources efficiently.
    Unfortunately for coral reefs, how- ever, a variety of human activities are causing worldwide degradation of shallow marine habitats by adding nutrients to the water. Agriculture, slash-and-burn land clearing, sewage disposal and manufacturing that creates waste byproducts all increase nutrient loads in these waters. Typical symptoms of reef decline are destabilized herbivore populations and an increasing abundance of algae and filter-feeding animals. Declines in reef communities are consistent with observations that nutrient input is increasing in direct proportion to growing human populations, thereby threat- ening reef communities sensitive to subtle changes in nutrient input to their waters.
The passage is primarily concerned with
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 主题题型：A．人类活动描述。本来正确，但局限于“algae in coral reefs”。B．正确。解释人类活动怎样对珊瑚礁造成威胁。C．解释珊瑚礁在贫营养水体中的衰退。说反了，应用“富营养”水体。D．文中不是珊瑚礁为自身创造食物，而是其中共生的藻类。E．描述两种物种的丰富。只是在文中附带提出。
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