Adrum can begin to vibrate at its natural frequency without being struck through:

a. refection

b. diffraction

c. refraction

d. resonance

answer is

Explanation:

If an external vibration matches the drum's natural frequency, it will induce the can to vibrate. This is an example of resonance.

It is analogous to pumping your legs in synchronization with a swing's natural frequency.

specific gravity is the density of a substance divided by the density of water. since (at standard temperature and pressure) water has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is usually very close to the same value as density

so "c" is the answer

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