The independent variable in the hypothesis "the longer a U. S. line worker has been employed at a U. S.-based assembly plant, the more difficult it is for that worker to find new employment when the assembly plant moves to Mexico" is .

Independent Variable

"The longer a U.S. line worker has been employed at a U.S.-based assembly plant..."

Explanation:

The independent variable is the element or variable that is independent of another variable.  In this case, "how difficult it is for the line worker to find new employment when the assembly plant moves to Mexico" a dependent variable, which depends on the length of time that the "U.S. line worker has been employed at a U.S.-based assembly plant," and not vice versa.

answer; synthetic production system;