at that time sea otters' population fell to the point that t

by consuming large numbers of sea urchins,sea otters act as a keystone species structuring the kelp forest community.4559

这题逻辑应该选哪个?为什么?
In the late 1980s,the population of sea otters in the North Pacific began to decline.There are two plausible explanations for the decline:predation,possibly by killer whales,or disease.Of these two,disease is the more likely,since a concurrent sharp decline in populations of seals and sea lions is believed to have been caused by disease,and diseases that infect these creatures are likely to be able to infect sea otters also.
Which of the following,if true,most seriously weakens the reasoning?
(A) Killer whales in the North Pacific usually prey on seals and sea lions but will,when this food source is scarce,seek out other prey.
(B) There is no indication that the sea otter population at any North Pacific location declined in the 1980s because of substantial numbers of sea otters migrating to other locations.
(C) Along the Pacific coast of North America in the 1980s,sea otters were absent from many locations where they had been relatively common in former times.
(D) Following the decline in the population of the sea otters,there was an increase in the population of sea urchins,which are sea otters' main food source.
(E) The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.

sea otters have very little fat.in order to adapt to the cold water temperature during the winter period,they have very dense furred skin,which acts as great insulator.4546

off the coast of western north america,however,sea otters feed on the urchins and can prevent them from becoming too abundant,thereby keeping the kelp forests intact.4547 想知道prevent them from becoming 是个固定用法吗?

off the coast of western north america, however,sea otters feed on the urchins and can prevent them from becoming too abundant, thereby keeping the kelp forests intact. 4547 什么意思?