In 1998 the United States Department of Transportation received nearly

1998:
- 10,000 consumer complaints about airlines
- Number of complaints per 100,000 passengers was say 1000

1999:
20,000 consumer complaints about airlines
- Number of complaints per 100,000 passengers also more than doubled so became say 2000

Number of complaints per 100,000 passengers has doubled too so an increase in the number of passengers cannot explain the increase in number of complaints from 10k to 20k. More percentage of people are complaining.

Conclusion: Despite the United States airline industry’s serious efforts to improve performance in these areas, passenger dissatisfaction with airline service increased significantly in 1999.

What can weaken it?
It does seem that more people are complaining so it would be natural to think that more people are unhappy. But what if more complaints came because people were encouraged to complain and talk about the problems they were facing. Since the industry was making efforts to improve, what if feedback was taken specifically from people? These people, though may have been unhappy before too but may not have complained.

A. Although the percentage of flights that arrived on time dropped slightly overall, from 77 percent in 1998 to 76 percent in 1999, some United States airlines’ 1999 on-time rate was actually better than their 1998 on-time rate.

Doesn't matter whether performance has improved or not. We need to know whether people's satisfaction has worsened.

B. The number of passengers flying on United States airlines was significantly higher in 1999 than in 1998.

As discussed before, number of passengers is irrelevant.

C. Fewer bags per 1,000 passengers flying on United States airlines were lost or delayed in 1999 than in 1998.

Doesn't matter whether performance has improved or not. We need to know whether people's satisfaction has worsened.

D. The appearance in 1999 of many new Internet sites that relay complaints directly to the Department of Transportation has made filing a complaint about airlines much easier for consumers than ever before.

Correct. Filing a complaint has become easier so more people are doing it. Even those who were unhappy before but didn't bother to file are now filing.

E. Although the number of consumer complaints increased for every major United States airline in 1999, for some airlines the extent of the increase was substantial, whereas for others it was extremely small.

We are talking about the industry as a whole.

Answer (D)