Given below are two statements. One is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A) :$ An electron in a certain region of uniform magnetic field is moving with constant velocity in a straight line path.
Reason $(R) :$ The magnetic field in that region is along the direction of velocity of the electron.
In the light of the above statements,choose the correct answer from the options given below.

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