# The amplitude of a simple pendulum , oscillating in air with a small spherical bob, decreases from 10 cm to 8 cm in 40 seconds .Assuming that Stokes law is valid , and ratio of the coefficient of viscosity of air to that of carbon dioxide is 1.3 , the time in which amplitude of this pendulum will reduce from 10 cm to 5 cm in carbondioxide will close to (ln 5=1.601, ln 2=0.693)

$(a)\;231\;s\qquad(b)\;208\;s\qquad(c)\;161\;s\qquad(d)\;142\;s$