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The population inversion in helium-neon laser is produced by 

(a) photon excitation 

(b) chemical reaction 

(c) inelastic atomic collision 

(d) electron excitation

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The population inversion in helium-neon laser is produced by 

(a) photon excitation 

(b) chemical reaction 

(c) inelastic atomic collision 

(d) electron excitation

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