Lets take Total dollar amount expenses : 100$
Total number of people who took trip : 100
As per passage : half of the total dollar amount spent on the transportation was airfare and large majority did not travel by airplane.

So Airfare other trans.
Total dollar amt 50$ 50$ half of the total dollar amount spent on the transportation was airfare
No. of people 40 60 large majority did not travel by airplane, so taking 40 < half
Clearly the average of transportation expenses of Airfare (50/40) greater than (50/60).

(A) Most of those who traveled by airplane did so because the airfare to their destination was lower than the cost of other available means of transportation.
-- Incorrect , out of scope.

(B) Most of those who traveled by airplane did so because other means of transportation to their destinations were unavailable.
-- Incorrect , out of scope this is not given.

(C) Per mile traveled, those who traveled by airplane tended to spend more on transportation to their destination than did those who used other means of transportation.
-- Incorrect , out of scope.

(D) Overall, people who did not travel by airplane had lower average transportation expenses than people who did.
-- Correct , this is actually inline with the inference.

(E) Those who traveled by airplane spent about as much, on average, on other means of transportation as they did on airfare.
-- Incorrect as we already know that people on average spend more on airplane than other transportation , both average can't be same , as number of people are different.

Ans D

I have a few questions:
1.) On the sentence "and about half of the total dollar amount spent on transportation was for airfare"... I realize that this sentence is joined by a comma conjunction, but it leads me to ask...total dollar amount spent on what? I realize that it is "total dollar amount spent on trips for pleasure" based on the first sentence, but why does the second sentence get away with not specifying this? To me, it is vague. Am I missing a grammar rule here?
-To clarify, for Choice C, distance traveled is basically irrelevant because we care about total costs (e.g., who cares if you traveled 2 miles or 200,000 miles on plane vs. any other form of transport, there is still a greater spend on planes)? Also, to confirm, Choice C COULD BE TRUE... but it does not have to be, right?
-To clarify, Choice E cannot be true because basically this would not explain the difference in total spent (traveling by other forms than airplane would then have to be more than airplane spend because then you have the vast majority of people + the few that are spending the most on airplanes)?
Thank you!