Question

wind tunnels: a test of form and function
look up at the sky, and you are likely to see airplanes, helicopters, and other types of aircraft soaring high above you. what you might not realize is that these aircraft all had to pass a series of tests to be considered safe enough to fly. one tool that researchers use when testing aircraft is a wind tunnel. this essential machine helps scientists improve the design of aircraft.
how wind tunnels work
wind tunnels are tubes that vary in size depending upon the object being tested. airplanes require huge wind tunnels. at one end of the tunnel is a fan blowing air. the fan can be set to different speeds and directions. an object is held in place in the tube, either with physical restraints or with the use of magnets. then the fan blows air around the object, creating an effect similar to flying.
wind - tunnel experiments help us understand how an object reacts to different wind situations. the tests can show how wind speed and direction affect the object while taking off, flying, and landing. researchers search for hidden flaws in the design of objects such as an aircraft. to help do this, they often add smoke or colored dye to the air. then the researchers photograph how the smoke or dye moves over the object while the fans are on.
which sentence from the passage best supports the inference that wind tunnels are an important tool when testing aircraft?
a wind - tunnel experiments help us understand how an object reacts to different wind situations.
b to help do this, they often add smoke or colored dye to the air.
c nasa constantly tests new designs, materials and shapes in an effort to improve its spacecraft.
d but there are other reasons why these machines are so essential.

Explanation:

The first option directly states that wind - tunnel experiments help understand how an object reacts to different wind situations, which is crucial for aircraft testing. This shows the importance of wind tunnels in aircraft testing. The other options do not as directly support the idea that wind tunnels are important for aircraft testing.

Answer:

A. Wind - tunnel experiments help us understand how an object reacts to different wind situations.