Question $1$:

The equation of exchange becomes a theory in which the quantity of money explains prices based on the assumption of classical economists that:

a$.$ both the velocity of money and real output are constant for an economy in the short run equilibrium.

b$.$ both the velocity of money and real output are constant for an economy in the long run equilibrium.

c$.$ the velocity of money is constant, whereas the quantity of output changes rapidly in an economy.

d$.$ the real output is constant whereas the velocity of money changes constantly in an economy.