Assertion $(A)$: When a proton and a neutron enter into a transverse magnetic field with equal speeds,they trace circular paths of equal radii.
Reason $(R)$: In a transverse magnetic field,the period of revolution of a charged particle in a circular path is directly proportional to the mass of the particle.

• A
  Both $(A)$ and $(R)$ are correct and $(R)$ is the correct explanation of $(A)$.
• B
  Both $(A)$ and $(R)$ are correct but $(R)$ is not the correct explanation of $(A)$.
• C
  $(A)$ is correct but $(R)$ is not correct.
• D
  $(A)$ is not correct but $(R)$ is correct.