In which of the following situations is there zero net force on the body?
a. an airplane flying due north at steady 120 m/s and at a constant altitude
b. a car driving straight up a hill with a 30* slope at a constant 90 km/h
c. a hawk circling at a constant 20 km/s at a constant height of 15m above an open field;
d. a box with a slick, frictionless surface in the back of a truck as the truck accelerates forward on a level road at (5 m/s squared)?