On the Average, People with Great Wealth Rate Themselves as Only

On the average, people with great wealth rate themselves as only slightly happier than people with much less wealth. One reason why these results are difficult to interpret is that

A) wealth is correlated with advanced age, and age is positively correlated with happiness.
B) so far, all the surveys have sampled only small numbers of people.
C) the rating scales may mean different things to different people.
D) longitudinal studies show that gaining wealth has little effect on happiness at first, but a bigger effect later on.