Coal is used as a source of energy in many manufacturing processes. Assume a new mining technology that raises the productivity and increase the supply of coal and reduced the price of coal. This would cause
A A) the short-run aggregate supply curve to shift to the right.
B B) the short-run aggregate supply curve to become flatter.

C) the short-run aggregate supply curve to shift to the left.
D
D) the short-run aggregate supply curve to become nearly vertical at all levels of output.