*$_Mass flow of granular solid (M) through a circular opening of dia, D follows?

• A. M ‚Äö√ √π ‚Äö√ √∂D
• B. M ‚Äö√ √π D¬¨‚â§
• C. M ‚Äö√ √π D¬¨‚â•
• D. M ‚Äö√ √π D