The index fossils are fossils that are found in the rock strata of certain geologic period, but they are also fossils that are very common and can be found in numerous places around the globe.
Because they can be found all over the globe, and they are indicative of certain geologic period, the geologists have started to use them as a simple way of determining the age of the rock strata. While this method can not provide the geologists with an accurate year for the formation of the rocks, it is still able to determine the relative age of the rocks. By determining the relative age with such ease by using the index fossils, the geologists are able to immediately make conclusions about the rock strata, have basic information about it, and largely be aware of the processes that formed them and influenced them.
D) because it is under all of the Earth's mass so it is pressing it together constantly... think of ooblek when you're shaping it together with your hands.
