Choose the correct option: 

Linear equations in two variables can be expressed as ____________.

ax + by = c  [ a,b,c are constants and x,y are variables]

$a{x}^2 + bx + c = 0$ [ a,b,c are constants and x,y are variables]

ax + by + c = 0  [ a,b,c are constants and x,y are variables]

$a{x}^3 - bx + c = 0$ [ a,b,c are constants and x,y are variables]