Assertion: As the work function of a material increases by some mechanism, it requires greater energy to excite the electrons from its surface.
Reason: A plot of stopping potential Vs versus frequency v for different metals has a greater slope for metals with greater work functions.

a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is not the correct explanation of A
c) A is true but R is false
d) A and R are false