A submarine is at the surface of the ocean. It begins its descent at a rate of 100 feet per minute. Write a linear model in the form d(t) =mt + b where d(t) represents the depth of the submarine relative to sea level, in feet, after t minutes into its descent. Be sure to include "d(t) =" in your answer. (Note: when the submarine is below sea level, its depth is a negative quantity)