Research Looking at the Relationship Between Income Per Person and Happiness

Research looking at the relationship between income per person and happiness indicates that an increase in a country's level of output per capita will:

A) generally have some effect on happiness in that country.
B) always decrease happiness in that country.
C) always increase happiness in that country.
D) increase happiness in that country if output per capita is relatively low.
E) no discernible change was found.