Final answer:
Milton Friedman and Edmund Phelps challenged the short-term tradeoff between inflation and unemployment suggested by the Phillips Curve in the late 1960s, demonstrating that in the long term, this tradeoff is not stable as the curve can shift, negating the supposed policy choices between inflation and unemployment rates.
Step-by-step explanation:
In the late 1960s, economists Milton Friedman and Edmund Phelps argued that a tradeoff between inflation and unemployment existed only in the short term. Initially, the Phillips Curve was used to describe an inverse relationship where policymakers believed they could choose either low inflation with high unemployment or high inflation with low unemployment. However, this appeared to break down during the 1970s as the Phillips Curve shifted; the notion of a sustainable tradeoff between inflation and unemployment was challenged, leading to the understanding that in the long run, there is no stable tradeoff as the curve can shift based on expectations and other economic factors.