The answer is :
A. Always
Also
If two equations have different slopes but equivalent y-intercepts, they will have one solution and that will be the point where the y-intercept is. If two equations have different slopes and different y-intercepts, then there will be one solution where those two lines meet. If two equations have the same slope but different y-intercepts, the lines will be parallel, and there is no possible intersection point. And if two equations have equal slopes and equal y-intercepts, these lines will have an infinite amount of solutions, because if the equations are one the same line, every single point on that line is a solution to the system.