求问这道题背后的逻辑思路，怎么也想不明白了

笨蛋鸟

In the late 1980s, the population of sea otters in the North Pacific Ocean began to 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 true, most seriously weakens the reasoning?

• A
  
  Killer whales in the North Pacific usually prey on seals and sea lions but wil, when this food source is scarce, seek out other prey.
  
  
• B
  
  There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980s.
  
  
• 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 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.
  
  
  

长丝弓

个人理解：
题干限定2种可能：杀人鲸捕食或疾病。并下结论说是疾病导致的。削弱就是寻它因，找另一种可能，最切题的不是疾病，当然就是杀人鲸。所以A正确