The answer is C. The only sphere that is entirely made up of gasses (and that surrounds the earth) is the atmosphere. The gasses in the atmosphere are held in place by the gravity of earth.
<h2>Petroleum that is worthwhile extracting is usually found trapped in layers of permeable rocks by other layers of impermeable rock, but more recently reserves of gas and oil are being extracted from shale which is an impermeable rock but is porous in the sense that there are spaces (pores) within its structure in which ...</h2>
