D, E, and F are on sides BC, AC, and AB, respectively, of Triangle ABC such that DE║AB, DF║AC, and BC║EF. Prove that D, E, and F are the midpoints of the sides of Triangle ABC.

answer: the answer is   graph (a)

large crystals result from very slow cooling.

evaporation of water: this would produce an evaporite which is a sedimentary rock. like a salt pan in a desert. not possible to have volcanic rocks originate this way.

crystalization in molten earth: slow cooling over time. (hundreds to millions of years)

crystalization at earth's surface: this results in fast cooling. (a few hours to a few months or years)

crystalization from cooling off hot water: that would be a hydrothermal deposit. while associated with volcanics they are not volcanic rocks themselves and usually occur fairly rapidly.