Given below are the two statements: one is labeled as Assertion $(A)$ and the other is labeled as Reason $(R)$.
Assertion $(A)$: There is a considerable increase in covalent radius from $N$ to $P$. However,from $As$ to $Bi$,only a small increase in covalent radius is observed.
Reason $(R)$: Covalent and ionic radii in a particular oxidation state increase down the group.
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
  Both $(A)$ and $(R)$ are true but $(R)$ is not the correct explanation of $(A)$
• C
  $(A)$ is true but $(R)$ is false
• D
  Both $(A)$ and $(R)$ are true and $(R)$ is the correct explanation of $(A)$