Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: If oxygen ion $(O^{-2})$ and hydrogen ion $(H^{+})$ enter normal to the magnetic field with equal momentum,then the path of $O^{-2}$ ion has a smaller curvature than that of $H^{+}$.
Reason $R$: $A$ proton with same linear momentum as an electron will form a path of smaller radius of curvature on entering a uniform magnetic field perpendicularly.
In the light of the above statement,choose the correct answer from the options given below.

• A
  $A$ is true but $R$ is false
• B
  Both $A$ and $R$ are true but $R$ is $NOT$ the correct explanation of $A$
• C
  $A$ is false but $R$ is true
• D
  Both $A$ and $R$ are true and $R$ is the correct explanation of $A$