Official Explanation
The passage describes two different year-over-year increases in airline passenger complaints: both the absolute number of complaints and the rate of complaints more than doubled from 1998 to 1999. From these facts, the author of the passage concludes that passenger dissatisfaction with airline service significantly increased in the same period.
Choice D is the correct answer: it weakens the argument because it presents a scenario in which the increase in complaints and in the rate of complaints could merely be the result of an easier means of filing complaints, not an actual increase in passenger dissatisfaction.
Choice A and Choice C are incorrect because neither presents a scenario that bears directly on the claim that passenger dissatisfaction increased from 1998 to 1999.
Choice E is incorrect: the fact that some airlines experienced a smaller increase than others does not change the fact that all airlines experienced an increase and thus cannot weaken the argument. Choice B could be correct if the passage discussed only the change in the absolute number of complaints, since more passengers could account for more complaints without entailing an increase in dissatisfaction. But the passage also says that the rate of complaints increased, making Choice B incorrect.
Answer: D