In 2008, the worlds airlines reported an increase in the total number

Here's how each option stands:

A. The airline industry was a victim of the recession in 2008. [It's difficult to establish how the recession would have impacted the revenue negatively]

B. Total passenger miles were up in 2008. [The revenue should have increased in such a case]

C. Fuel costs remained constant from 2007 to 2008. [Doesn't negatively or positively impact as this is a static variable]

D. Passengers traveled shorter distances on less expensive flights in 2008. [Here is a comparative data that actually gives a valid reason for why the revenues could have dropped even though the numbers remained static]

E. No new aircraft were purchased by any carrier in 2008. [Again, this point negates a possibility but doesn't give insight into why there could be a drop in revenue]