Choose the correct assumption from the list below: a. The gravitational force acting on a differential volume element of fluid is a part of the total force that acts on a control volume. O b. The magnitude of the gravitational force acting on a differential volume element of fluid is equal to the total force acting on the control volume minus the magnitudes of the summation of pressure forces, viscous forces and those from reaction forces. O C. The gravitational force acting on a differential volume element of fluid is equal to its weight. O d. All assumptions are correct