Question

The Earth's Layers

Each layer of the Earth is different from the other layers in temperature, density, and composition. The crust is the outer layer and is relatively thin and rocky. It is made up of silicon, aluminum, calcium, sodium, and potassium. The next layer below the crust is the mantle. The mantle is a thick layer of hot liquid rock called magma. The mantle is mainly composed of silicon, oxygen, magnesium, iron, aluminum, and calcium, and it is about 1,800 miles thick. The core of the Earth is divided into the inner and outer core. The outer core is made of extremely hot liquid iron and nickel. It is about 1,425 miles thick. The inner core is composed of an incredibly hot solid iron-nickel core. The inner core remains solid because it is under such extreme pressure. It is about 750 miles in radius. The inner core may have a temperature up to about 13,000°F, which is hotter than the surface of the Sun.
5
Which of the following makes the best introductory sentence?
A.
Geologists use seismic waves to study the Earth's layers and draw conclusions about the Earth's core.
B.
The Earth is a giant sphere composed of four main layers: the crust, the mantle, the outer core, and the inner core.
C.
The crust of the Earth is thickest under mountains and is thinnest at the ocean floor.
D.
Scientists believe that the core of the Earth is much denser than the rest of the planet.

Answer 1

B

This sentence introduces the topic (the Earth's layers) and names the four layers that the paragraph then describes, making it the best introductory sentence.