[With some straws, she is making a musical pipe.]
1. When you blow across the top of "a)the straw", the air vibrates inside "b)the straw" and makes a sound.
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a)the straw b)the straw
Are they the specific straw or in generic use?
When "c)the straw" is shorter, the air loses less energy and vibrates faster.
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Does "c)the straw" mean the specific straw or is it in generic use?
Again, these uses of "the straw" are referring to a particular straw.
In this context, both "a) the straw" and "b) the straw" are referring to a generic straw. It means that any straw can be used to create a musical pipe. So, when you blow across the top of a straw, the air inside the straw vibrates and produces sound.
Similarly, "c) the straw" also refers to a generic straw. It means that when the length of any straw is shorter, less energy is lost and the air vibrates at a faster rate, resulting in a higher-pitched sound.