Assertion $(A)$: $A$ helium-filled balloon does not rise indefinitely in the air but halts after a certain height.
Reason $(R)$: Viscosity opposes the motion of the balloon. Choose any one of the following four responses:

• A
  If both $(A)$ and $(R)$ are true and $(R)$ is the correct explanation of $(A)$.
• B
  If both $(A)$ and $(R)$ are true but $(R)$ is not the correct explanation of $(A)$.
• C
  If $(A)$ is true but $(R)$ is false.
• D
  If $(A)$ is false but $(R)$ is true.