how can we determine the following sets are convex or not? can it be enough to draw a 3d graph? or how can we apply the formula f( x + (1- )y) f(x) + (1- )f(y) for f(x1,x2)?

Which of the following sets are convex and which are not?

a. {(x1 x2) : x1^2+ x2^2 1}

b. {(x1, x2) : x1 = 1, |x2| 4)

c. {(x1, x2) : x2 — x1^2 = 0}

d. {(0,0), (0,1), (1,0), (1,1)}