Which of the following must be followed by the flow of a fluid (real or ideal)? (I) Newton's law of viscosity. (II) Newton's second law of motion. (III) The continuity equation. (IV) Velocity of boundary layer must be zero relative to boundary. (V) Fluid cannot penetrate a boundary.

• A. I, II, III
• B. I, II, V
• C. II, IV, V
• D. II, III, V

Answer: (D)