What was Niels Bohr's explanation for the observation of atomic spectra?
A. Electrons could only move in discrete energy steps within an atom.
B. Only certain photons with the correct energy could excite the quanta in the nucleus.
C. Nucleons could be excited by different electron energies.
D. Any photon could excite an electron.
E. Electrons could not move in an atom.
Answer: A
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