Susan's cell phone plan includes unlimited texting. The amount she pays for texting each month can be described by the function y=20, where y is the number of dollars spent on texting as a function of x, the number of texts

Answer Choices:
1.It is linear because the rate of change is zero
2.It is linear because the rate of change is increasing
3.It is nonlinear because the rate of change is zero
4.It is nonlinear because the rate of change is increasing.