The rate law of a reaction $A+B \to$ product is rate $=K [A]^n[B]^m$. On doubling the concentration of A and halving the concentration of B. Thr ratio of new rate to the earlier rate of reaction will be :

Question

$(A)\;n-m \\ (B)\;2^{n-m} \\(C)\;\large\frac{1}{2^{m+n}} \\(D)\;m+n$