GWD逻辑题

xuromeo

in the late 1980,the population of sea otters in the north pacific ocean began do decline .Of the two plausible explanations for the decline---increased predation by killer whales or disease ---disease is the more likely.After all,a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution related disease,which could have spread to sea otters,whereas the population of killer whales did not change noticeably.
which of the following ,if ture,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
为啥选A？求翻译和解释 谢谢！

xuromeo

其他选项如下
(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.