The answer is C Why Please explain I thought it would be A S

The answer is C. Why? Please explain. I thought it would be A? Something with a vertical line test. Confused?

Ok here is the reason for C y is called a function of x if for every one value of x there exists one unique y but in this case this does not happen. as for y = -ve and +ve (same magnitude value) x can be same ... basically this graph is a circle. so you can imagine for 1 value of x there are 2 y\'s. I guess even A is right.