Question

Manhole covers are made circular:

• A. to strengthen cover
• B. to make the entry convenient
• C. for architectural reasons
• D. to prevent falling of the cover into the manhole

Hint:

Circular objects have uniform diameter, making them safest for covering circular openings like manholes.

Answer 1

The Correct Option is D

Step 1: Understanding manhole design.
Manholes provide access to underground sewer lines for inspection and maintenance. The cover design is crucial for safety and functionality.
Step 2: Reason for circular shape.
A circular cover cannot fall through its opening regardless of its orientation, unlike square or rectangular covers which can slip diagonally.
Step 3: Analyzing the options.
(A) Incorrect, strength is not the main reason.
(B) Incorrect, entry convenience is unrelated to shape.
(C) Incorrect, architectural aesthetics are not the priority.
(D) Correct, circular shape prevents accidental falling into the manhole.
Step 4: Conclusion.
Manhole covers are circular primarily to prevent falling of the cover into the manhole.

Answer 2

This question is about the geometry behind manhole cover design, so the reasoning should focus on how a circular shape behaves differently from other shapes when it is tilted or turned over an opening of the same size.

1. To strengthen cover: Strength is influenced more by material and thickness than by the outline shape of the cover, so this is not the primary reason for choosing a circle.
2. To make the entry convenient: A worker's entry is not made noticeably easier by a circular opening compared to a square one of similar size, so convenience does not explain the choice either.
3. For architectural reasons: Manhole covers are functional utility items, not decorative features, so appearance is not the governing design factor.
4. To prevent falling of the cover into the manhole: A circle has the same width measured through its centre in every direction, so it can never be tilted and slipped diagonally through its own opening, unlike a square or rectangular cover whose diagonal is longer than its side.

Because only the circular shape has this constant-width property that stops the cover from ever passing through its own hole, safety against the cover falling in is the real reason behind the design.

Therefore, the correct answer is to prevent falling of the cover into the manhole.