PLEASE SOLVE QUESTION C,D ONLY. BUT PLEASE SOLVE IN DETAIL. I NEED TO KNOW THE TECHNIQUE FOR THESE TYPE OF PROBLEM

8] Which of the following sets are convex and which are not? a. {(x1,x2):x12+x223}. b. {(x1,x2,x3):x1+2x21,x12x32}. c. {(x1,x2):x23x12=0}. d. {(x1,x2,x3):x2x12,x1+2x2+x34}. e. {(x1,x2):x1=3,x24}. f. {(x1,x2,x3):x3=x2,x13}