“Beating a dead horse” means?

1. A Option A: To uselessly dwell on a subject far beyond its point of resolution
2. B Option B: An obvious, pressing issue left unaddressed due to its sensitive nature.
3. C Option C: To speak the truth, even to the point of being blunt and rude
4. D Option D: To work late into the night, alluding to the time before electric lighting

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