In this question, we are given two copper wires, A and B, with different lengths but the same area of cross-section. We need to find the ratio of the resistivity of wire A to the resistivity of wire B.

The resistivity of a material is a characteristic property that determines how well it resists the flow of electric current. It is denoted by the symbol ρ (rho).

The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area. The formula for resistance is R = ρ * (L/A), where R is the resistance, ρ is the resistivity, L is the length of the wire, and A is the cross-sectional area.

Given that the area of cross-section is the same for both wires, we can simplify the equation to R = ρ * L.

Since the resistance of a wire is directly proportional to its length, and wire A has a length of I and wire B has a length of 21, we can write the ratio of their resistances as R(A)/R(B) = I/21.

Now, we need to consider the ratio of their resistivities. Since the resistivity is not affected by the length or cross-sectional area, the ratio of