A wind tunnel is used for testing planes or plane models. In a wind tunnel air is blown ov

er a plane or a model placed in a test section. Wind tunnel test sections are different in size depending on airflow speed requirements. Some low-speed tunnels have test sections large enough for a complete small plane. In the very high speed tunnels used for space testing, however, the model may be as small as a pencil.

There are two ways of feeding air to the test section. In the constant flow tunnel, airflow produced by electric fans is forced through the tunnel to the test section, then through another tunnel section back to the starting point for reuse. The other way involves storing high-pressure air in a box, sending it out by a controlling device to pass through the tunnel and test section into another box. This way is usually used for a very short period of high-speed airflow.

Wind tunnels are usually used to test planes before they are flown. Problems in a plane can be found when it meets the high-speed airflow in the tunnel. For example, if the wings are built so that they would weaken at high speeds, this fact can be discovered in the tunnel, thus saving many lives and millions of dollars.

In a wind tunnel, scientists also find out how different planes will act at different speeds. They find out such things as how the air dives by the wing surfaces, how slowly a plane can fly before it starts to drop, and how fast it can climb.

Some wind tunnel sections are larger than others because ______.

A．they are used for large planes

B．they can produce a stronger wind

C．they are built to conduct a high-speed test

D．the air in them flows at a lower speed