Except for the sun, the moon looks like the biggest object in the sky. Actually it is one of the smallest, and only looks big because it is so near to us. Its diameter(直径) is only 2,160 miles (3,339 km) ,or a little more than a quarter of the diameter of the earth.
Once a month, or more exactly, once every 29.5 days, at the time we call “full moon”, its whole disc looks bright. At other times only part of it appears bright, and we always find that this is the part which faces towards the sun, while the part racing away from the sun appears dark. People could make their pictures better if they kept this in mind — only those parts of the moon which are lighted up by the sun are brighter. This shows that the moon gives no light of its own. It only throws back the light of the sun, like a huge mirror hung in the sky.
Yet the dark part of the moon’s surface is not completely black; usually it is just light enough for us to be able to see its shape, so that we speak of seeing “the old moon in the new moon’s arms”. The light by which we see the old moon does not come from the sun, but from the earth. We know well how the surface of the sea or of snow, or even of a wet road, may throw back uncomfortably much of the sun’s light on to our faces. In the same way the surface of the whole earth throws back enough of the sun’s light on to the face of the moon for us to be able to see the parts of it which would otherwise be dark.
72. Why is the dark part of the moon not completely black?
A. The sun shines on the moon’s surface.
B. The earth throws back sunlight on to the moon.
C. The moon throws back the light from the sun.
D. The moon has light of its own.
73. The underlined word “disc” in the second paragraph refers to _______.
A. a round plate                                 B. a round record
C. the moon that reflects sunlight         D. the moon which looks like a round plate
74. By saying “the old moon in the new moon’s arms” , we mean a time when ______.
A. the two moons are closely linked
B. the new moon is at its brightest
C. the moon is partly bright and partly dark
D. the new moon is hugging the old moon
75. Which of the following is true according to the text?
A. The moon which appears round at its brightest is called full moon.
B. The moon’s diameter is exactly one fourth of that of the earth.
C. The light by which we see the old moon comes from the sun
D. The part of the moon which is not lighted by the sun is completely dark.