ANS:B - the ratio of spans in two directions is less than 2

An R.C.C. roof slab is designed as a two-way slab if:

1. It supports live loads in both directions: Two-way slabs are designed to support live loads acting in both the longitudinal and transverse directions. This means that the slab is capable of distributing the applied loads effectively in two directions.
2. The ratio of spans in two directions is less than 2: Two-way slabs are characterized by having relatively small ratios of span lengths in the two orthogonal directions. Typically, the span length in the shorter direction is not more than double the span length in the longer direction. This ensures that the slab behaves as a two-way structural system and distributes loads efficiently.

Therefore, the correct condition for designing an R.C.C. roof slab as a two-way slab is:

• The ratio of spans in two directions is less than 2.

While the other conditions mentioned (the slab is continuous over two supports and the slab is discontinuous at edges) may also apply to certain design scenarios, they are not specific to the designation of a slab as a two-way slab. The key characteristic distinguishing a two-way slab is the ratio of spans in two directions being less than 2.