Or are they the fallen celestial beings, called the sons of God in Genesis 6, that came into the daughters of men, and begat giants?

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Which spirits are these?

I Peter 3:18 For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:

  19 By which also he went and preached unto the spirits in prison;

  20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.

Are "the spirits in prison", all those who drowned in the flood?

Or are "the spirits in prison", mentioned here, only the giants, the offsprings of the sons of God and the daughters of men?