Suppose that during the Great Depression long-run aggregate supply shifted left. To be consistent with what happened to the price level and output, what would have had to happen to aggregate demand?

The aggregate demand will fall

Explanation:

The aggregate supply measures the quantity of real GDP that can be supplied by in the economy at different price levels. it measures planned output if both prices and average wage rates can change, the Long run aggregate supply curve is assumed to be vertical (this means it remains constant when the general price level changes).

The leftward shift in aggregate supply means that at the same price levels the quantity supplied of real GDP has decreased. This is mostly due to natural disasters or other supply shocks like economic depression, when there is leftward shift in aggregate there would be fewer workers available to produce goods at any given price.