Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$ : The photoelectric effect does not take place,if the energy of the incident radiation is less than the work function of a metal.
Reason $R$ : Kinetic energy of the photoelectrons is zero,if the energy of the incident radiation is equal to the work function of a metal.
In the light of the above statements,choose the most appropriate answer from the options given below.

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