This question is probably asked over and over again, but I cannot come up with keywords that can clearly describe this question to a search engine.
One of my friends has this fill-in-the-blank question on their test:
"Why has student spending become a greater concern over the last few years? Because there is a trend of ____ use."
The correct answer is "credit card use," but my friend got it wrong for using the plural form and writing down "credit cards use," and my friend is curious as to why this is wrong. I cannot remember a quantitative grammar rule that defines this other than that my language intuition tells me it is wrong. Can anyone give a quantitative explanation?