4. McDonald was ying his crop duster horizontally at an altitude of LDIJD feet with a ground speed of Bi] feet per second. The plane passes directly over a corn field. His wife was in the field at the time and looked straight up to see it ying over but then lowers her gaze to follow the plane. At what rate is the angle of elevation of McDonalds wife's line of sight changing 3' seconds after the plane has passed overhead