How do scientists use index fossils to determine the relative ages of rock layers in other locations?
By comparing all the fossils in one rock layer they know the age of the fossil.
By locating other areas of the world where the same index fossils are found, scientists know the relative age of the rock in each area is the same.
By comparing fossils in many different rock layers the age of the rock can not be found.
By determining the positons of fossils in many different rock layers in one area they know the age of the rock.