Assertion $(A)$: Shape of the receptor changes after attachment of chemical messenger.
Reason $(R)$: Receptor does not regain original shape after removal of chemical messenger.
The correct answer is

• A
  $(A)$ and $(R)$ are correct and $(R)$ is the correct explanation of $(A)$
• B
  $(A)$ 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