A hollow enclosure is formed between two infinitely concentric cylinders of radii 1 m and 2 m respectively. Radiative heat exchange takes place between the inner surface of the larger cylinder (surface 2) and the outer surface of the smaller cylinder (surface 1) the radiating surfaces are diffuse and the medium in the enclosure is non-participating. The fraction of the thermal radiation leaving the surface and striking itself is

• A. 0.25
• B. 0.5
• C. 0.75
• D. 1