大全61，还是topic

Passage 61 (61/63)

Coral reefs are one of the most fragile, biologically complex, and diverse marine ecosystems on Earth. This ecosystem is one of the fascinating paradoxes 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. 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, hosts, and even for other members of the reef community. This symbiotic process allows organisms in the reef community to use sparse nutrient resources efficiently.

Unfortunately for coral reefs, however, 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 by-products 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 threatening reef communities sensitive to subtle changes in nutrient input to their waters.

1.b   The passage is primarily concerned with

(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（D）

(E) describing the abundance of algae and filter-feeding animals in coral reef areas

我选的A，我是根据第二段的unfortunately，我觉得实在强调后面的内容，答案D只是说了第一段的内容，我觉得不足以代表整个passage的中心意思。

大家认为呢？