D(t)=10+4.9cos(π6t) The function D defined above models the depth, in feet, of the water t hours after 12 A. M. in a certain harbor. Which of the following presents the method for finding the instantaneous rate of change of the depth of the water, in feet per hour, at 6 A. M. ?

Every weekday, mr. jones bikes from his home to his job. sometimes he rides along two roads, the long route that is shown by the solid lines. other times, he takes the shortcut shown by the dashed line. how many fewer kilometers does mr. jones bike when he takes the shortcut instead of the long route?