ANS:B - clear distance between the inner faces of the walls plus twice the thickness of the wall

The effective span of a simply supported slab is the clear span plus the effective depth of the slab. Explanation:

• The clear span is the distance between the supports or bearings of the slab.
• The effective depth of the slab refers to the distance from the extreme compression fiber to the centroid of the tensile reinforcement.

So, the effective span takes into account both the clear span (distance between supports) and the effective depth of the slab, which is crucial for calculating bending moments and deflections accurately. The other options provided do not accurately define the effective span of a simply supported slab:

• The distance between the centers of the bearings may include additional lengths beyond the effective span.
• The clear distance between the inner faces of the walls plus twice the thickness of the wall is not typically used to define the effective span. It's more related to the overall dimensions of the structure.
• The clear span plus effective depth of the slab is the correct definition of the effective span.

Therefore, the correct answer is: Clear span plus effective depth of the slab.