9) All of the following, expect one, are examples of how a trade union or professional association successfully restricts the supply of labour available. Which is the exception? A) Only licensed M.D.s (doctors) can practice medicine. B) Only licensed pipe-fitters can open a gas line. C) Only members of the Bar cala argue a case in court. D) Only individuals with a PhD can be employed at a university. E) The government imposes a minimum wage law