Answer:
B. Erosion and D. Index fossils are geologic features that help scientists to determine relative ages of rocks by their positions.
Step-by-step explanation:
Relative age is used in geology to determine the rocks age with respect to other rocks either older or younger. Two of the methods employed are erosion and index fossils.
Erosion creates gaps in the stratigraphic column called unconformities. The gaps are evident when some rock layers are missing from one column but present on the other columns in the same location. Thus, erosion is used to determine the age of the rocks. Index fossils method is used to match layers of rocks found in different places or areas. This means that if the layers have a similar index, then they are of the similar age.