The rate of decay of mass of a certain substance at time $t$ is proportional to the mass at that instant. The time during which the original mass of $m_{0}$ grams will be reduced to $m_{1}$ grams is (where $k$ is the constant of proportionality):

• A
  $\frac{1}{k} \log \left(\frac{m_{1}}{m_{0}}\right)$
• B
  $k \log \left(\frac{m_{0}}{m_{1}}\right)$
• C
  $k \log \left(\frac{m_{1}}{m_{0}}\right)$
• D
  $\frac{1}{k} \log \left(\frac{m_{0}}{m_{1}}\right)$