In a four bar planar mechanism shown in the figure, $AB= 5 \: cm, AD=4 \: cm$ and $DC=2 \: cm$. In the configuration shown, both AB and DC are perpendicular to AD. The bar AB rotates with an angular velocity of $10 \: rad/s$. The magnitude of angular velocity (in rad/s) of bar DC at this instant is

1. $0$
2. $10$
3. $15$
4. $25$