What do they mean in parts a and b ?

then that particular organisms become extinct.

what subject is this

the three components of a nucleotide are the nitrogenous base, the phosphate group and the five-carbon sugar.

they have identical fossils of organisms that have evolved over time.

both have animal fossils preserved in them due to weathering and erosion.

they contain different fossils because life on earth has changed through time.

the fossils in them were formed on the surface of earth due to exposure to sunlight.