Aresearcher is using a panel data set on n = 1000 workers over t 10 years (from 2001 through 2010) that contains the workers' earnings, gender, education, and age. the researcher is interested in the effect of education on earnings. determine whether each of the following is an example of unobserved person-specific or time-specific variables that are correlated with both education and earnings
a. unobserved ability
b. gdp growth
c. unobserved household environment
d. unobserved motivation
e. unemployment level