Which of the Following Factors Might Explain Why the Long-Run

Which of the following factors might explain why the long-run equilibrium number of firms can in some instances exceed the socially optimal number?

A) The appropriability effect (the increase in consumers surplus following entry is not "appropriated" by entrants) .
B) The feedback effect (an increase in the number of firms increases the competitiveness of the market) .
C) The business-stealing effect (entry reduces rival firms' profits,a social loss that entrants do not account for) .
D) The ratchet effect (the more profits the entrants earn,the more the stockholders expect them to earn in the future) .