Einstein Introduced a "Cosmological Constant" into His Formulation of the Structure

Einstein introduced a "cosmological constant" into his formulation of the structure of the universe on the basis of the general theory of relativity. How did he envision that this cosmological constant would manifest itself?

A) as antimatter that, by annihilating with real matter, would translate matter into energy, thereby maintaining a constant mass density in an condensing universe
B) as many "white holes" that would contribute matter to an expanding universe to maintain constant density, as required by the cosmological principle-a continuous creation universe
C) as a form of energy that, on its own, would make the universe expand-a form of antigravity
D) as an extra "gravity" that would hold the universe against continuous expansion