If a simple pendulum has significant amplitude only in the period between $t=0\text{s}$ to $t=\tau \text{s}$, then $\tau$ may be called the average life of the pendulum. When the spherical bob of the pendulum suffers a retardation (due to viscous drag) proportional to its velocity, with '$b$ ' as the constant of proportionality, the average life time of the pendulum is (assuming damping is small) in seconds:

1. A
   \frac{0.693}{b}
2. B
   b
3. C
   \frac{1}{b}
4. D
   \frac{2}{b}