Four uniform spheres, with masses mA = 40 kg, mB = 35 kg, mC = 200 kg, and mD = 50 kg, have (x, y) coordinates of (0, 50 cm), (0, 0), (-80 cm, 0), and (40 cm, 0), respectively. In unit-vector notation, what is the net gravitational force on sphere B due to the other spheres?