To the Degree That the Plumbers and Steamfitters Union Ensures

To the degree that the Plumbers and Steamfitters union ensures that through its rigorous training requirements for prospective plumbers are qualified to do the work, it is

A) shifting the supply of plumbers to the left.
B) shifting the supply of plumbers to the right.
C) shifting the demand for plumbers to the right.
D) kinking the supply of plumbers to make it vertical at the level of the constraint.