From which part the light column is divided into beams in grazing incident technique?

• A. At the hypotenuse of the system‚Äö√Ñ√∂‚àö√ë‚àö¬•s precision prism
• B. At base of the system‚Äö√Ñ√∂‚àö√ë‚àö¬•s precision prism
• C. After reflecting from the test surface
• D. At perpendicular of the system‚Äö√Ñ√∂‚àö√ë‚àö¬•s precision prism