Which of the following are non$-$linear programming models?

$($a$)$ Maximize profit $=3x_1+5x_2+99x_3$ subject to the contraints

$x_{1}10$

$x_{2}5$

$x_{3}18$

$($b$)$ Maximize profit $=25x_1+30x_2+8x_1{x}_2$ subject to the contraints

$x_1=8$

$x_1+x_2=12$

$x_1-x_2=11$

$($c$)$ Maximize protit $=3x_1+4x_2$

subject to the contraints

$x_1^2-5x_2=8$

$3x_1+4x_2=12$

$($d$)$ Maximize profil $=18x_1+5x_2+x_2^2$

subject to the contraints