____ formula is used for calculating the depth of foundation.

• A. Gordon‚Äö√Ñ√∂‚àö√ë‚àö¬•s
• B. Rankine‚Äö√Ñ√∂‚àö√ë‚àö¬•s
• C. WH Smith‚Äö√Ñ√∂‚àö√ë‚àö¬•s
• D. Falcon