The work of the paleolithic artists stands out today because

The work of the Paleolithic artists stands out today because the artistshad an understnading of depth and movement that was not seen again until many thousands of years later

 it is endangered, discovery and exposure to visitors and outside air cause problems that threaten the very existence of those paintings.

option c (nickel) is not found in earth's crust. the earth's crust basically contains eight (8) elements namely: oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium (listed in order of abundance).