What option best fills the gap labelled 12

The passage below has gaps. Immediately after each gap, four options are provided. Choose the most appropriate option for each gap. 

In modern mining, there is a network of __6__ [A. way B. tunnels C. paths D. avenues] and passages, with openings and walls which in the dim light seem very bewildering. Right across the wall at the end of the inky black passage, the __7__ [A. miners B. toilers C. workers D. foreman] approach. The coal __8__ [A. front B. slope C. obstacle D. face] is neatly cut __9__ [A. slit B. gash C. bare D. aperture]. It extends level to a height of rather less than three feet. The rays of the miner’s lamp shine on steel roof __10__ [A. beams B. supports C. spurs D. stays]. Thousands of feet of rock and earth are pressing down on those steel plates and __11__ [A. props B. wedges C. poles D. stakes], and the miners have to crawl through the small, deep opening in the wall to get to the coal in the mine that we are considering, the __12__ [A. gash B. hole C. seam D. opening] is nearly four feet thick and is being worked by a combination of __13__ [A. digging B. manpower C. machinery D. spade] and explosives. A mechanical __14__ [A. slit B. gash C. knife D. mesh] saws into the coal to a depth of four or five feet, thus separating it from its base. Explosives pushed into holes drilled at intervals in the coal are __15__ [A. fused B. extinguished C. detonated D. stuck] by remote control and blow the separated section of the coal to pieces.