Pauley graphs the change in temperature of a glass of hot tea over time. He sees that the function appears to decrease quickly at first, then decrease more slowly as time passes. Which best describes this function?

It is linear because the graph decreases over time.
It is linear because there is both an independent and a dependent variable.
It is nonlinear because linear functions are increasing functions.
It is nonlinear because linear functions increase or decrease at the same rate