A is a circle around the origin and a line through the origin, so they'll intersect in two places; we can skip the detailed calculation.
B is the intersection of a parabola and a line; they can intersect in zero, one or two places depending on the details. Let's check


Discriminant,

Negative discriminant, zero real solutions for B.
System C is again a parabola and a line; we proceed similarly:




Zero discriminant, exactly one real solution for C.