An R.C.C. roof slab is designed as a two way slab if A. It supports live loads in both directions B. The ratio of spans in two directions is less than 2 C. The slab is continuous over two supports D. The slab is discontinuous at edges

The main advantage of a two-way slab is that it is more efficient in terms of material usage than a one-way slab. This is because the load is distributed more evenly across the slab, which means that less material is required to support the load.

However, two-way slabs are also more complex to design and construct than one-way slabs. This is because the loads must be carefully calculated to ensure that the slab is not overloaded. Additionally, two-way slabs require more reinforcement than one-way slabs.

Overall, two-way slabs are a versatile and efficient type of concrete slab that can be used in a variety of applications. They are particularly well-suited for buildings with open floor plans, as they allow for greater flexibility in the layout of the space.

Here is a brief explanation of each option:

• Option A: It supports live loads in both directions. This is the correct answer, as a two-way slab is designed to carry loads in both the longitudinal and transverse directions.
• Option B: The ratio of spans in two directions is less than 2. This is not necessarily the case, as two-way slabs can be designed with any ratio of spans.
• Option C: The slab is continuous over two supports. This is not necessarily the case, as two-way slabs can be designed to be either continuous or discontinuous over supports.
• Option D: The slab is discontinuous at edges. This is not necessarily the case, as two-way slabs can be designed to be either continuous or discontinuous at edges.