$17.$ A key challenge in modeling three$-$phase flows is:

A$)$ Simplifying the geometry

B$)$ Ignoring the presence of solids

C$)$ Accounting for interactions between all three phases

D$)$ Assuming identical phase velocities

$18.$ In agitated vessels, the primary mechanism for mass transfer enhancement is:

A$)$ Sedimentation

B$)$ Agitation

C$)$ Filtration

D$)$ Centrifugation

$19.$ The minimum fluidization velocity in fluidized beds is critical for:

A$)$ Ensuring solid particles remain stationary

B$)$ Initiating particle movement

C$)$ Maximizing energy consumption

D$)$ Reducing heat transfer

$20.$ The main advantage of pneumatic conveying systems is:

A$)$ Their ability to transport solids over long distances efficiently

B$)$ Low maintenance requirements

C$)$ High precision in solid flow rate control

D$)$ Minimal energy consumption