Which of the following did Keynes think was a macroeconomic problem?

a)

The equilibrium level of output can fall below the potential level of output.

b)

The equilibrium level of output can never be equal to the potential level of output.

c)

The potential level of output and the equilibrium levels of output can be the same.

d)

Over time, the potential level of output may be pulled down to the equilibrium level of output