The photoelectric behaviour of $\mathrm{K}, \mathrm{Li}, \mathrm{Mg}$ and $\mathrm{Ag}$ metals is shown in the plot below. If light of wavelength $400 \mathrm{nm}$ is incident on each of these metals, which of them will emit photoelectrons? [Planck's contant $h=6.626\times {10}^{-34}\mathrm{Js}$; velocity of light $c=3\times {10}^8 {\mathrm{ms}}^{-1}; 1\mathrm{eV}=1.6\times {10}^{-19} \mathrm{J}$]