Assertion $(A)$: Adenine and guanine are present in $RNA$ as purine bases.
Reason $(R)$: The base uracil is present in $RNA$.
The correct option among the following is:

• A
  $(A)$ is true,$(R)$ is true and $(R)$ is the correct explanation for $(A)$.
• B
  $(A)$ is true,$(R)$ is true but $(R)$ is not the correct explanation for $(A)$.
• C
  $(A)$ is true but $(R)$ is false.
• D
  $(A)$ is false but $(R)$ is true.