# If a simple pendulum has significant amplitude (up to a factor of 1/e of original) only in the period between $t = Os$ to $t = \tau s$, 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 :
( A ) $b$
( B ) $\frac{0.693}{b}$
( C ) $\frac{2}{b}$
( D ) $\frac{1}{b}$