When you blow across the opening of a partially filled 2-L soda pop bottle you hear a tone.

(a) If you take a sip of the pop and blow across the opening again, does the tone you hear have a higher frequency, a lower frequency, or the same frequency as before?

(b) Choose the best explanation from among the following:

I. The same pop bottle will give the same frequency regardless of the amount of pop it contains.

II. The greater distance from the top of the bottle to the level of the pop results in a higher frequency.

III. A lower level of pop results in a longer column of air, and hence a lower frequency.