Which of the following is the correct expression for the relation between Einstein s coefficients A and B?

• A. ( frac{8 pi v^3 h}{c^3} )
• B. ( frac{8 pi v^2 h}{c^3} )
• C. ( frac{8 pi v^2 h}{c^2} )
• D. ( frac{8 pi hv}{c^3} )