pendulum, T $\propto \sqrt{l}$
$5 \times {10^{ - 4}} = \frac{1}{2}\frac{{{r_1} - {r_2}}}{1}$
$\because$ change in length $\Delta l=r_{1}-r_{2}$
$5 \times {10^{ - 4}} = \frac{1}{2}\frac{{{r_1} - {r_2}}}{1}$
$r_{1}-r_{2}=10 \times 10^{-4}$
$10^{-3} \mathrm{m}=10^{-1} \mathrm{cm}=0.1 \mathrm{cm}$


$(A)$ Restoring force is directly proportional to the displacement.
$(B)$ The acceleration and displacement are opposite in direction.
$(C)$ The velocity is maximum at mean position.
$(D)$ The acceleration is minimum at extreme points.
Choose the correct answer from the options given below :