Assertion $(A) :$ The distance between two polynucleotide chains in $\text{dsDNA}$ remains constant.
Reason $(R) :$ Always a purine comes opposite to pyrimidine or vice versa.

• A
  Both $A$ and $R$ are true and $R$ is the correct explanation of $A$.
• B
  Both $A$ and $R$ are true but $R$ is $\text{NOT}$ the correct explanation of $A.$
• C
  $A$ is true but $R$ is false
• D
  $A$ is false but $R$ is true