For logarithm of time fitting method, the coefficient of consolidation is detained from ________

• A. (c_v= frac{(T_v)_{70} d^3}{t_{70}} )
• B. (c_v= frac{(T_v)_{80} d^2}{t_{80}} )
• C. (c_v= frac{(T_v)_{90} d^4}{t_{90}} )
• D. (c_v= frac{(T_v)_{50} d^2}{t_{50}} )