Assertion $(A) :-$ When a small amount of strong acid or strong base is added to pure water,its $pH$ undergoes a change.
Reason $(R) :-$ Addition of an acid or base increases the degree of ionisation of water.

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