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Object distances are always negative according to the Cartesian sign convention
In the blank should go of friction.
Answer:
Thermal/Heat energy, kinetic energy, light energy, & Electromagnetic energy
For a cylinder that has both ends open resonant frequency is given by the following formula:

Where n is the resonance node, v is the speed of sound in air and L is the length of a cylinder.
The fundamental frequency is simply the lowest resonant frequency.
We find it by plugging in n=1:

To find what harmonic has to be excited so that it resonates at f>20Hz we simply plug in f=20 Hz and find our n:

We can see that any resonant frequency is simply a multiple of a base frequency.
Let us find which harmonic resonates with the frequency 20 Hz:

Since n has to be an integer, final answer would be 323.