Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A) :$ Emission of electrons in photoelectric effect can be suppressed by applying a sufficiently negative electric potential to the photoemissive substance.
Reason $(R) :$ $A$ negative electric potential, which stops the emission of electrons from the surface of a photoemissive substance, varies linearly with frequency of incident radiation.
In the light of the above statements, choose the most appropriate answer from the options given below:

• A
  $(A)$ is false but $(R)$ is true.
• B
  $(A)$ is true but $(R)$ is false.
• C
  Both $(A)$ and $(R)$ are true and $(R)$ is the correct explanation of $(A).$
• D
  Both $(A)$ and $(R)$ are true but $(R)$ is not the correct explanation of $(A).$