If x₁=f(P₁,P₂) and x₂=φ(P₁,P₂), then the two commodities are substitutes if
A. (δx₁/δP₂)<0,(δx₂/δP₁)>0
B. (δx₁/δP₂)>0,(δx₂/δP₁)<0
C. (δx₁/δP₂)>0,(δx₂/δP₁)>0
D. (δx₁/δP₂)<0,(δx₂/δP₁)<0