C. It requires active transport because the use of energy is necessary, the natural and opposite flow to this, would be from high to low concentration that requires little to no energy (i.e passive diffusion).
D.
If the vibrations have a low frequency, the sound is heard as a __low-pitched sound_____. If the vibrations have a high frequency, then the sound is heard as a __high-pitched sound_____.
Explanation:
High pitched sound is of higher frequency because they make the air molecules vibrate much faster than low-pitched sound. Due to their high frequency, their wavelength is shorter (Remember wavelength and frequency are inversely proportional). Vice versa is true to the low-pitched sound. They vibrate air molecules slower than high-pitched sound. Their wavelength is longer and frequency smaller.
Learn More:
For mor on pitch of sound check out;
brainly.com/question/61859
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Ya its B i think :):):)):
