EA wind tunnel （风洞) is used for teasing planes or plane models. In a wind tunnel air is

EA wind tunnel (风洞) is used for teasing planes or plane models. In a wind tunnel air is blown over a plane or a model placed in a test section (区).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 is produced by electric fans. It 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 (卷入) storing high-pressure air in a box, sending it out by a controlling device (装置) 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 (become weak) at high speeds, this fact can be discovered in the tunnel, saving many lives and millions of dollars.

In a wind tunnel, scientists also find out how different planes will act 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.

第52题：Some wind tunnel sections are larger than others because ________.

[A] they are used large planes

[B] stronger wind can be produced in them

[C] they are built to meet a higher speed test

[D] the wind used in them flows at a lower speed