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Threshold frequency for a metal is ${10}^{15}$ Hz. Light of $\stackrel{0}{\mathrm{A}}$ falls on its surface. Which of the following statements is correct

(a) No photoelectric emission takes place

(b) Photo-electrons come out with zero speed

(c) Photo-electrons come out with ${10}^{3}$ m/sec speed

(d) Photo-electrons come out with ${10}^{5}$ m/sec speed