Air is a colourless, tasteless, invisible gas that surrounds the planet Earth. It is everywhere on the planet. An "empty" drinking glass and an "empty" room, to give two examples, are not really empty. Each is filled with air. When the glass is filled with water, the water pushes the air out of the glass.
Air as a gas, has no definite (一定的) shape, hut, because it is matter, it takes up space. It is easy to prove that air is something that takes up space. Stuff a dry handkerchief into the bottom of a glass so that it will not fall out when the glass is turned upside down. Push the upside-down glass, hold straight up and down, into a jar of water till the glass is completely covered. When the glass is taken out of the water, the handkerchief will be dry. The air inside the glass took up space and kept the water from coming in.
What's the article mainly about？

Air is a kind of gas.
B. Air is invisible.
C. Air is everywhere.
D. Air takes up space.