Ultra-violet light is shone on a zinc surface and photoelectrons are emitted. The sketch graph shows how the stopping potential ${V_s}$ varies with frequency $f$.

Planck’s constant may be determined from the charge of an electron $e$ multiplied by

A.     the $x$-intercept.

B.     the $y$-intercept.

C.     the gradient.

D.     the area under the graph.

C