According to one of the videos, what is the relationship between labor union membership and middle class incomes in the US, and what drives that relationship? a. As union membership has gone down over time, the average income of middle class workers has also gone down because labor unions balance the power of workers with corporations b. As union membership has gone down over time, the average income of middle class workers has gone up because labor unions raise costs for firms and force them to cut wages c. As union membership has gone up over time, the average income of middle class workers has gone down because labor union bosses are often corrupt and frequently fight for changes that are in their best interests d. As union membership has gone up over time, the average income of middle class workers has also gone up because labor unions are recognized as an important partner to firms in managing their workforces