To those who believe in the valuation obtained by the dividend discount model $($DDM$),$ which of the statements below correctly describe the relationship between the fair value implied by the DDM and the actual price observed in the stock exchange?

a$.$ The believers of DDM would expect that the fair value of the stock to move towards and become closer to the actual price over time.

b$.$ None of the other choices are correct

c$.$ A stock is undervalued if the fair value is significantly above the actual price

d$.$ A stock is undervalued if the fair value is significantly below the actual price