Why can't rna adopt a b-form helical structure?

Under physiological condition, RNA exists in an A form helix structure unlike DNA which adopts a B form helix structure. The A conformation found in RNA is caused by the steric limitation imposed on RNA by its ribose components. The steric clash between the second and third hydroxyl groups in the ribose component of the RNA molecule makes the formation of B form by the RNA molecule to be unfavourable, therefore the RNA is forced to adopt the A conformation. This A conformation is also responsible for the more stable structure of the RNA molecule.