GMAS 8th Grade Science Practice Test

Earth is built in three broad shells from outside to inside: the crust, the mantle, and the core. The crust is the thin outer skin we live on, the mantle is a thick, rocky layer beneath it, and the core sits at the center and is divided into a liquid outer part and a solid inner part. The core is made of two regions, but they’re both part of the same core layer, so listing crust, mantle, and core names the three main interior layers.

Choosing crust, mantle, and outer core leaves out the inner core, which is still part of the core. The option that includes lithosphere mixes a tectonic-plate concept with the interior layers, and the one with atmosphere and hydrosphere refers to surface air and water, not the interior structure. So the three main layers are crust, mantle, and core.