If $a, b,$ and $c$ are constants, which of the following is a linear inequality?

GATE · 1987 · CS · Question 1 subparts

1. A.

   $ax+bcy=0$

2. B.

   $ax^{2}+cy^{2}=21$

3. C.

   $abx+a^{2}y \geq 15$

4. D.

   $xy+ax \geq 20$