For the four circles $\mathrm{M}, \mathrm{N}, \mathrm{O}$ and P , following four equations are given : Circle $\mathrm{M}: \mathrm{x}^2+\mathrm{y}^2=1$ Circle $\mathrm{N}: \mathrm{x}^2+\mathrm{y}^2-2 \mathrm{x}=0$ Circle $\mathrm{O}: \mathrm{x}^2+\mathrm{y}^2-2 \mathrm{x}-2 \mathrm{y}+1=0$ Circle P : $x^2+y^2-2 y=0$ If the centre of circle M is joined with centre of the circle N , further centre of circle N is joined with centre of the circle O , centre of circle O is joined with the centre of circle P and lastly, centre of circle $P$ is joined with centre of circle $M$, then these lines form the sides of a :