Assertion (A) : A man standing on a completely frictionless surface can propel himself by whistling. Reason (R) : If no external force acts on a system, its momentum cannot change.

(a)

Both A and R are true, and R is the correct explanation of A

(b)

Both A and R are true, but R is not a correct explanation of A

(c)

A is true, but R is false

(d)

A is false, but R is true