In a linear programming problem, which of the following represents a correct objective function?

A$)$ MaxZ$=5xy$

B$)$ MinZ$5x^2+2y^2$

C$)$ Max$3x+3y+\frac{1}{3}z$

D$)$ Min$\frac{x_1+x_2}{x_3}$

a$.A$

b$.$ B

c$.C$

d$.D$