Resistance depends on path length and cross-sectional area

Steps:

• Use formula R = ρ L / A, where L is distance between opposite faces and A is cross-sectional area perpendicular to L.
• Assume dimensions: length 3x, width 2x, height x; pairs P (sep. x, A=6x²), Q (sep. 3x, A=2x²), R (sep. 2x, A=3x²).
• Calculate: R_P = ρ x / 6x² = ρ/(6x); R_Q = ρ (3x) / 2x² = (3ρ)/(2x); R_R = ρ (2x) / 3x² = (2ρ)/(3x).
• Compare: R_Q (1.5ρ/x) > R_R (0.67ρ/x) > R_P (0.17ρ/x), so maximum for Q.

Why B is correct:

• Maximum resistance occurs for longest L (3x) and smallest A (2x²), per R = ρ L / A.

Why the others are wrong:

• A: R_P is minimum due to shortest L and largest A.
• C: Resistances differ because L and A vary by direction.
• D: R_R is intermediate, not maximum.

Final answer: B