# The rate law for a reaction between the substances A and B is given by rate = $k [A]^n [B]^m$. On doubling the concentration of A and halving the concentration of B, the ratio of the new rate to the earlier rate of the reaction will be as:
( A ) $2^{(n-m)}$
( B ) $(m+n)$
( C ) $(n-m)$
( D ) $\frac{1}{2^{m+n}}$