Let us take a brief look at the planet on which we live. As Earth hurtles through space at

a speed of 70,000 miles an hour, it spins, as we all know, on its axis, which causes it to be flattened at the Poles. Thus if you were to stand at sea level at the North or South Pole, you would be 13 miles nearer the centre of the earth than if you stood on the Equator.

The earth is made up of three major layers — a central core, probably metallic, some 4000 miles across, a surrounding layer of compressed rock, and to top it all a very thin skin of softer rock, only about 20 to 40 miles thick — that's about as thin as the skin of an apple, talking in relative terms.

The pressure on the central core is unimaginable. It has been calculated that at the centre it is 60 million pounds to the square inch, and this at a temperature of perhaps 10,000 degrees Fahrenheit. The earth's interior, therefore, would seem to be of liquid metal — and evidence for this is given by the behavior. of earthquake.

When an earthquake occurs, shock waves radiate from the centre just as waves radiate. outwards from the point where a stone drops into a pond. And these waves pulsate through the earth's various layers. Some waves descend vertically and pass right through the earth, providing evidence for the existence of the core and an indication that it is fluid rather than solid. Thus, with their sensitive instruments, the scientists who study earthquakes, the seismologists, can in effect X-ray the earth.

Iceland is one of the most active volcanic regions of the world. And it was to Iceland that Jules Verne sent the hero of his book A Journey to the Centre of the Earth. This intrepid explorer clambered down the opening of an extinct volcano and followed its windings until he reached the earth's core. There he found great oceans, and continents with vegetation. This is conception of a hollow earth we now know to be false. In the 100 years since Jules Verne published his book, the science of vulcanology, as it is called, has made great strides. But even so the deepest man has yet penetrated is about 10,000 feet. This hole, the Robinson Deep mine in South Africa, barely scratches the surface; so great is the heat at 10,000 feet that were it not for an elaborate air-conditioning system, the miners working there would be roasted. Oil borings down to 20,000 feet have shown that the deeper they go, the hotter it becomes.

The temperature of the earth at the centre is estimated to be anything between 3,000 and 11,000 degrees Fahrenheit. Some scientists believe that this tremendous heat is caused by the breaking-down of radio-active elements, which release large amounts of energy and compensate for the loss of heat from the earth's surface. If this theory is correct, then we are all living on top of a natural atomic powerhouse.

The outer layer of the earth is compared to the skin of an apple because ______.

A．it is about 20 to 40 miles thick

B．it is thin in proportion to the earth's mass

C．it is softer than the central core

D．it is thinner than the surrounding layer