Assume you are interested in measuring the economic progress of immigrants over time. However, you only have data following "cohorts" of immigrants. When using such data, which of the following would lead to an incorrect measurement of the progress of immigrants in the labor market?

Immigrants who are doing well in the labor market might be less likely to return to their countries of origin

Immigrants who are not doing well in the labor market might be more likely to be missed by government surveys

Those who entered the US in 1980 had higher earnings than those who entered in 1990

a and c

a and b