Several proposals have been presented to increase the extraction ratio in a room and pillar system to 50% from 33%. The pillars are long compared with width, so the problem is essentially two-dimensional. 

Proposal 1; Increase the ratio of opening width to pillar width from 0.5 to 1.0 by increasing entry width to 30 ft (9 m) from 20 ft (6 m) while decreasing pillar width. Spacing is held constant.

Proposal 2; Increase the ratio of opening width to pillar width from 0.5 to 1.0 by keeping pillar width constant while increasing entry width to 40 ft (12 m).

Proposal 3; Increase the ratio of opening width to pillar width from 0.5 to 1.0 by keeping entry width constant while decreasing pillar width to 20 ft (6 m). Which proposal is preferable from the rock mechanics view and why?