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d) For more than 30 years, the prevailing view of the formation of our moon has been the "giant impact hypothesis".

a) The precursors to the current four rock planets – Mercury, Venus, Earth and Mars – appear to have been dozens of smaller bodies known as "planetary embryos".

c) According to the giant impact hypothesis, our moon formed as the result of the last of a series of "giant impact" mergers between planetary embryos that eventually formed the Earth.

b) In this last collision, one embryo was nearly Earth-sized and the other approximately Mars-sized.