Regardless of the danger that the roof might collapse, the fireman entered the building.

1. A Option A: Regardless of the danger that the roof might collapse, the fireman entered the building.
2. B Option B: Irrespective of the danger that the roof might collapse, the fireman entered the building.
3. C Option C: Disregarding of the danger that the roof might collapse, the fireman entered the building.
4. D Option D: Irregardless of the danger that the roof might collapse, the fireman entered the building.

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