For second moment of area (area moment of inertia) of plane sections, identify the correct dependence of units and typical expressions (m^4, cm^4, mm^4) based on the chosen length and area units.

Introduction:
The second moment of area (area moment of inertia) quantifies how area is distributed about an axis and affects bending stiffness (EI) and deflections. Its units reflect both the area element and the squared distance from the axis, leading to length units to the fourth power.

Given Data / Assumptions:

• I = ∫ y^2 dA conceptually (no calculus required here), where dA is area and y is distance to the axis.
• Area units and length units chosen for the coordinate system determine the final I units.

Concept / Approach:

Dimensionally, I has units of length^4 because dA contributes length^2 and the squared distance contributes another length^2. Therefore, depending on whether metres, centimetres, or millimetres are used, units become m^4, cm^4, or mm^4 respectively.

Step-by-Step Solution:

Verification / Alternative check:

Typical steel design tables provide section properties in cm^4 or mm^4; finite element software often allows unit systems where the same dimensional reasoning applies.

Why Other Options Are Wrong:

• A–D are individually correct statements; the best comprehensive choice is E.

Common Pitfalls:

• Confusing area moment of inertia (length^4) with mass moment of inertia (masslength^2).
• Forgetting to convert all dimensions consistently when switching unit systems.

Final Answer:

All the above.