Quantity of money demanded varies:

If wages and prices were flexible, they would adjust quickly to changes in demand and supply, eliminating the trade-off between inflation and unemployment. This would result in a vertical long-run Phillips curve rather than the upward sloping short-run version, indicating that flexibility undermines the relationship that the Phillips curve illustrates.

High competition typically leads to lower prices and can drive wages down, which would not support an upward-sloping Phillips curve. Instead, it tends to encourage a more stable price level and potentially a flatter Phillips curve, where inflation does not rise significantly even as unemployment decreases.

The downward-sloping aggregate demand (AD) curve indicates that as prices decrease, the quantity of goods demanded increases. While this relates to general economic principles, it does not explain the upward slope of the short-run Phillips curve, which specifically focuses on the relationship between inflation and unemployment rather than the shape of the AD curve.