Assertion : The first derivative of a vector of constant magnitude (either zero or a nonzero) is perpendicular to the vector itself. Reason : Scalar product of two vectors obeys commutative law.

• A

  If both assertion and reason are true and reason is the correct explanation of assertion.
• Correct answer
  B

  If both assertion and reason are true but reason is not the correct explanation of assertion.
• C

  If assertion is true but reason is false.
• D

  If both assertion and reason are false.

Hence, the correct answer is option: 'B'.