For a ribbed slab A. Clear spacing between ribs shall not be greater than 4.5 cm B. Width of the rib shall not be less than 7.5 cm C. Overall depth of the slab shall not exceed four times the breadth of the rib D. All the above

The correct answer is D. All the above.

The clear spacing between ribs shall not be greater than 4.5 cm to ensure that the concrete in the ribs is adequately compacted. The width of the rib shall not be less than 7.5 cm to provide adequate support for the concrete in the slab. The overall depth of the slab shall not exceed four times the breadth of the rib to ensure that the slab is not too thin and can support the loads placed on it.

Here are some additional details about each of the options:

• Option A: The clear spacing between ribs shall not be greater than 4.5 cm. This is to ensure that the concrete in the ribs is adequately compacted. If the spacing is too great, the concrete in the ribs may not be compacted properly, which could lead to problems with the strength and durability of the slab.
• Option B: The width of the rib shall not be less than 7.5 cm. This is to provide adequate support for the concrete in the slab. If the width is too narrow, the concrete in the slab may not be adequately supported, which could lead to problems with the strength and durability of the slab.
• Option C: The overall depth of the slab shall not exceed four times the breadth of the rib. This is to ensure that the slab is not too thin and can support the loads placed on it. If the depth is too great, the slab may be too weak to support the loads placed on it, which could lead to problems such as cracking and buckling.