![](https://cos-cdn.shuashuati.com/pipixue-wap/2020-1230-1107-56/ti_inject-812ce.png)
What can we learn from the passage?
A.
It is very expensive to build a wind tunnel.
B.
Wind tunnels are very essential in testing planes.
C.
Scientists still have a lot to learn about wind tunnels.
D.
There are new ways to feed air to the test section.