The argument says that half the dollar amount of TE was spent on airfare and the other half on other means of transportation. But the people travelling by air was much less than people travelling by other means.

Says $5000 spent on air and $5000 on other means. People travelling by air = 100 and people travelling by other means = 500.

So average expense per person by air = 5000/100 = $50 and by other means = 5000/500 = $10

Since this is an inference question, we are trying to find the option that must be true. Between C and D - it is not necessary that "per mile" expense needs to be greater for airfare than other means. As long as average expense for airfare is higher for airfare we are good. Thus D wins over C.
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