Final answer:
The size of the crystals in an igneous rock can help identify how it was formed. Large crystals form in rocks that cool slowly deep within the Earth, while fine grains are a result of rapid cooling near or at the surface.
Step-by-step explanation:
The size of the crystals in an igneous rock can indicate how the rock was formed because the cooling rate of the magma or lava affects the crystal size. Option a is correct: The longer the rock takes to cool, the larger crystals can grow inside it. Slow cooling, which occurs deep within the Earth, allows crystals to grow larger, forming coarse-grained intrusive or plutonic rocks. Conversely, rapid cooling usually occurs near the Earth's surface, resulting in fine-grained extrusive or volcanic rocks. Characteristics of these rocks include the size of the mineral grains, whether the grains are large enough to be seen without magnification, and the interlocking of crystal edges, resembling jigsaw puzzle pieces.