Terminal velocity ($v_t$) is the velocity at which the gravitational downward force equals the aerodynamic drag force on a free-falling particle in still air. For vertical pneumatic conveying, the upward conveying air velocity ($v_{\text{air}}$) must be strictly greater than the particle's terminal velocity ($v_{\text{air}} \gt v_t$) to generate net positive aerodynamic lift. Thus, the terminal velocity of the material is lower than the air velocity.

Option analysis:

• Lower: Correct, particle terminal velocity must be lower than air velocity to allow upward transport.
• Higher: If terminal velocity exceeded air velocity, particles would settle and choke the pipe.
• Equal: The particles would remain stationary/suspended in mid-air (choking threshold).
• Zero: Physical particles have non-zero terminal velocity.

The three classical empirical laws governing energy requirements in milling and size reduction are derived from Walker's differential equation ($dE/dL = -K L^{-n}$):

• Kick's law ($n=1$): Energy is proportional to size reduction ratio, applicable to coarse grinding.
• Rittinger's law ($n=2$): Energy is proportional to new surface area created, applicable to fine grinding.
• Bond's law ($n=1.5$): Energy is proportional to square root of surface-to-volume ratio, intermediate grinding.
  Fick's laws (A) govern molecular diffusion and mass transfer, not size reduction.
  
  Option analysis:
• Includes Fick's law, which is incorrect.
• Includes Fick's law, which is incorrect.
• Includes Fick's law, which is incorrect.
• Correct, includes (B) Kick's law, (C) Rittinger's law, and (D) Bond's law.