A metal has a threshold frequency of ( 1.5 × 10¹⁵ ) Hz. If a photon of wavelength 180 nm hits the metal, causing an electron to be ejected, what will be the velocity of the electron?

a) ( 3.53 × 10⁶ ) m/s
b) ( 4.68 × 10⁶ ) m/s
c) ( 5.91 × 10⁶ ) m/s
d) ( 7.84 × 10⁶ ) m/s