What is the radius of the Mohrcircle

Question

At a point in two-dimensional stress system $\sigma _{x}$= 100 N/mm², $\sigma _{y}$ =$\\tau _{xy}$  = 40 N/mm². What is the radius of the Mohrcircle for stress drawn with a scale of: 1 cm = 10 N/mm²  ?                                            

(a) 3 cm                                   (b) 4 cm

(c) 5 cm                                   (d) 6 cm