Geologists determine absolute age by comparing the amounts of radioactive and nonradioactive elements in rock. The radioactive, or
, elements decay into nonradioactive elements. Because this rate of decay is
for each element, geologists can compare the amounts of stable and unstable elements to determine how much of the
elements have formed over time.

answer and explanation;

volcanic rocks are igneous rocks which form from lava, molten rock which has been thrust out of a volcano onto the surface of a rocky body such as an asteroid, planet, or dwarf planet.  

plutonic rocks are igneous rocks which form from subterranean chambers of molten rock or magma. plutonic rocks make up the base of continental crust as well as oceanic crust. their slow rate of formation causes them to be coarse grained since there is plenty of time for large crystals to form before the magma solidifies into rock.

the textural difference is that plutonic rocks are more coarse grained than volcanic rocks. plutonic rocks are coarse grained while volcanic are fine grained ; glassey or fragmental. since fine and coarse are adjectives the decision as to whether a rock is fine or coarse grained is generally made on the bases of whether or not the individual crystals can be seen in most if the rock.

i think the answer is a.keratin is the pigment responsible for skin color.