Well, I believe you got all of them right.
A. buying presents for your friend's birthdays is a cultural thing, because it is something that's been done for a long time and is part of human culture
B. buying an iPod because everyone has one is an example of a trend, because it is something popular that everyone does
C. buying a used car to save money has to do with prices and buying something that is still good, but for less money
