Reading Comprehension

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.

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

[spoiler]OA - B. But can we substantiate the first part of B ie 'how human activities are posing a threat'. It is threat or the threat has already led to negative effects. From the passage, it appears that human activities have already affecting the coral reefs. This lead to believe that first part of A ie 'effects of human activities' is better though the 2nd part is incorrect.[/spoiler]

Pls help

thanks
Patanjali

I would say you are thinking too much.
choice B says: " are posing " it just means it still i shappening at the moment.
its a fact stated.
where as A is so wrong you can reject it outright.

A is talking about effects of human activities on ALGAE whereas B is talking about threat posed to coral reefs by human activities .

"Posing threat" - As said by rohu you are thinking too much on this .

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

So it is ok if we are given with option which says "posing threat" .