Why do scientists focus their attention on sedimentary rock? How do you know?

A It is a type of rock that builds up slowly. "Koobi Fora is made up of sedimentary rock, a type of rock that forms when sediment, or sand and soil, builds up in layers."
B
It is the primary rock found in Koobi Fora. "All lakes, rivers and streams carry sediment, such as soil and sand."
C
It is where many fossils are located. "Bones of ancient humans and other animals stay buried in the rock and can become fossils."
D
It contains a special rock called tuff. "Over time, it is packed together into a special kind of rock called tuff."