Final answer:
A shift of the aggregate demand curve from the vertical segment to the horizontal segment would cause both an increase in real output and an increase in the average price level.
Step-by-step explanation:
A shift of the aggregate demand curve from the vertical segment to the horizontal segment would lead to both an increase in real output and an increase in the average price level. When the aggregate demand curve shifts from being vertical to horizontal, it means that the economy is operating at its full employment level, resulting in an increase in real output. At the same time, as the aggregate demand increases, it puts upward pressure on prices, leading to an increase in the average price level.