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The work functions of cesium (Cs) and lithium (Li) metals are 1.9 eV and 2.5 eV, respectively. If we incident a light of wavelength 550 nm on these two metal surface, then photo-electric effect is possible for the case of

1. Li only
2. Cs only
3. Neither Cs nor Li
4. Both Cs and Li

Correct answer: Cs only

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