4. Which of the following is not an explanation for the slight widening of the income distribution in the United States since 1970?

a. The increase in technology has decreased the demand for unskilled workers and increased the demand for skilled workers, causing the gap between their wages to widen.

b. An increase in international trade has decreased the demand for unskilled workers and increased the demand for skilled workers, causing the gap between their wages to widen. C. An increase in discrimination against unskilled immigrant labor has reduced the demand for unskilled workers and caused an increase in the gap between the wages of unskilled and skilled workers.