Which of the following is not an equation of a circle?

Select one:
a. \( 2 x^{2}+y^{2}-2 x+2 y=1 \)
b. \( x^{2}+y^{2}=4 \)
c. \( 3 x y+y^{2}+2 x-3=0 \)
d. \( 2 x^{2}+2 y^{2}+3 x=0 \)