Brick classification — which statement correctly describes second-class bricks as per common building practice? Choose the most accurate description.

Introduction / Context:
Bricks are graded by their burning quality, strength, and surface finish. First-class bricks are uniformly well-burnt with sharp edges and give a clear metallic ring. Second-class bricks are of moderate quality and often slightly under-burnt, making them suitable for less critical work where a perfect finish is not required.

Given Data / Assumptions:

• We compare acoustic response, burning degree, and shape/finish.
• Standard building materials terminology is assumed.
• The choice targets the defining trait of second-class bricks.

Concept / Approach:
Second-class bricks generally show minor distortions, chips, or surface irregularities and may be slightly under-burnt. They do not usually produce the clear metallic ring of first-class bricks and lack the sharp arrises and uniform colour typical of premium quality.

Step-by-Step Solution:

Verification / Alternative check:
Textbook definitions highlight slightly under-burnt and less regular geometry for second-class bricks versus first-class quality standards.

Why Other Options Are Wrong:

• Dark brown colour: Not a defining criterion of class.
• Metallic ring: Characteristic of first-class, not second-class.
• Well burnt with uniform shape: Again, first-class property.

Common Pitfalls:
Judging solely by colour; sound test and edge sharpness are stronger indicators of class.

Final Answer:
They are under-burnt (slightly underfired) and of moderate quality