When photons incident on a metallic surface electric current is produced. The work function of a metal is given by `[phi_0 = hc / lambda_0 = h(upsilon_0 )]` ,where `(lambda_0, c , h , upsilon_0)` have their usual meaning The graph given below shows the variation in stopping potential with frequency `upsilon` of the incident radiation of the materials A and B. : Which material has higher work function?