Gravity is a pair of forces of attraction between every
two objects. Period !
It doesn't matter whether they're moving together, moving apart,
moving gracefully in synchrony, or at rest. The gravitational forces
of attraction between them are always there.
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.
It seems that you have missed the necessary options for us to answer this question so I had to look for it. Anyway, the answer to the given statement is FALSE. Pigments are NOT always made of natural materials. Pigments can either be man-made or the one that occurred naturally as what you see in our environment. Hope this helps.