Why Do Archaeologists Use a Datum Point Rather Than Simply

Why do archaeologists use a datum point rather than simply measuring from the ground surface?

A) The datum point provides a universal reference point that can be used across any archaeological site, allowing archaeologists to easily compare data between excavations.
B) While vertical provenience could easily be measured from the ground surface, obtaining accurate horizontal provenience would be much more difficult without a datum point.
C) The ground surface does not have the same elevation consistently across a site while a datum point provides a fixed reference.
D) Use of a datum point is an archaeological tradition established in the early 20th century; while it serves no useful purpose, it is an example of how outdated excavation methodology is still embedded in archaeology today.