A small coin of mass `80g` is placed on the horizontal surface of a rotating disc. The disc starts from rest and is given a constant angular acceleration `alpha=2rad//s^(2)`. The coefficient of static friction between the coin and the disc is `mu_(s)=3//4` and cofficient of kinetic friction is `mu_(k)=0.5`. The coin is placed at a distance `r=1m` from the centre of the disc. The magnitude of the resultant force on the coin exerted by the disc just before it starts slipping on the disc is
A. `({((u_(s)g)^(2)/(r))+alpha^(2)}^(1//2))/(4pialpha)`
B. `({((u_(s)g)^(2)/(r))-alpha^(2)}^(1//2))/(4pialpha)`
C. `({((u_(s)g)^(2)/(r))-alpha^(2)}^(1//2))/(2pialpha)`
D. `({((u_(s)g)^(2)/(r))+alpha^(2)}^(1//2))/(2pialpha)`
