Why would a historian use a peimary source when studing about a historical event

Nwhich situations is the principle of cross-cutting relationships useful in determining relative age? check all that apply. a fault breaks through sedimentary layers. sedimentary layers form and then are tilted. igneous rock forms between sedimentary layers. sedimentary layers form over millions of years in a lake bottom. lava flows form on top of one another from several eruptions over time.