<u>A. The photons emitted when an electron drops down an energy level.
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<h2>Explanation:</h2>
When a source emits electromagnetic radiation then the spectrum is called as emission spectrum. It is emitted during the process when an atom makes its transition of energy levels to ground state.
Ground state is the state where atom possesses the lowest energy. When an atom or molecule comes down from higher energy level to lower energy level,it emits the spectrum which is called as emission spectrum.
Resonance in air columns is used in a variety of musical woodwind instruments. Something like a flute for example uses resonance to make sound. Not sure what instruments you are talking about unless you provide a list.
