In conductor sizing practice (using circular mils and feet), is it correct to express electrical resistivity in the customary unit Ω·cmil/ft (ohm–circular-mil per foot)? Provide the most appropriate evaluation of this unit statement.

Introduction / Context:
Resistivity describes how strongly a material opposes current flow. In SI it is commonly given in Ω·m. However, electricians and wire tables in North America often use circular mils (cmil) and feet (ft) to size conductors. This question checks whether Ω·cmil/ft is a legitimate customary unit for resistivity in that context.

Given Data / Assumptions:

• Conductor cross-section sometimes specified in circular mils (1 circular mil = diameter in mils squared).
• Length measured in feet.
• We are discussing material property ρ and the design constant used in wire tables.

Concept / Approach:
Ohm’s law for a uniform conductor is R = ρ * (L / A). If A is expressed in circular mils and L in feet, then ρ must carry units that cancel A and L to produce ohms. Therefore, a consistent customary unit is Ω·cmil/ft. In many cable tables, a material constant k is tabulated in Ω·cmil/ft at a stated temperature (for copper near 20 °C, k is about 10.4 Ω·cmil/ft). Designers compute R by R = k * L / A, with L in feet and A in circular mils.

Step-by-Step Solution:

Verification / Alternative check:
Compare with SI: if A is in m^2 and L in m, then ρ is Ω·m. Unit systems change the numeric value but not the physics; the formula remains R = ρ * L / A.

Why Other Options Are Wrong:

• Ω/ft^2 (option b) would make R dimensionless when used in R = ρ * L / A; it is inconsistent.
• “Only Ω·m” (option c) ignores accepted industry practice in AWG/cmil systems.
• cmil/Ω·ft (option d) inverts units and would not yield ohms from R = ρ * L / A.

Common Pitfalls:
Confusing resistivity (material property) with resistance (component property), and mixing SI with customary units without converting. Also, writing the order as cmil·Ω/ft or Ω·cmil/ft is equivalent multiplicatively, but clarity favors Ω·cmil/ft.

Final Answer:
Valid — Ω·cmil/ft is a correct customary unit for resistivity used with circular-mil and feet measurements.