A 2.5 m thick coal seam lying at an average depth of 100 m has been developed by bard and pillar method. The width of the square pillars is 30 m (centre to centre) and the gallery width is 4 m. The average density of the overlying strata is 26 kN/m3 and the pillar strength is 4500 kN/m2. Extraction ratio during the development of the pillar is
A. 0.129
B. 0.148
C. 0.218
D. 0.249
Answer: Option D
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