If the decay or disintegration constant of a radioactive substance is $\lambda$,then its half-life and mean life are respectively:

• A
  $\frac{1}{\lambda}$ and $\frac{\log_e 2}{\lambda}$
• B
  $\frac{\log_e 2}{\lambda}$ and $\frac{1}{\lambda}$
• C
  $\lambda \log_e 2$ and $\frac{1}{\lambda}$
• D
  $\frac{\lambda}{\log_e 2}$ and $\frac{1}{\lambda}$