Which of the Following Is Not an Explanation for Why

Which of the following is not an explanation for why better educated workers earn more,on average,than less educated workers in the United States?

A) Better educated workers have higher marginal productivities,on average.
B) Compensating differentials lower the wages of skilled workers relative to unskilled workers.
C) The United States tends to import goods produced with unskilled labor,which reduces the U.S.demand for unskilled labor.
D) The demand for skilled labor has risen over time relative to the demand for unskilled labor.