$17.$ In three$-$phase flows, the dynamics of gas$-$solid liquid contactors are crucial for:

A$)$ Designing efficient heat exchangers

B$)$ Predicting erosion in pipelines

C$)$ Optimizing chemical reaction rates

D$)$ All of the above

$18.$ Trickle bed reactors are commonly used in:

A$)$ Wastewater treatment

B$)$ Gas phase polymerization

C$)$ Hydroprocessing of heavy petroleum fractions

D$)$ Production of synthetic rubber

$19.$ Agitated vessels are used for gas$-$solid liquid mixtures to:

A$.$ Decrease the reaction time

B$.$ Increase the pressure drop

C$.$ Enhance mixing and mass transfer

D$.$ Reduce energy consumption

$20.$ The pressure drop in packed beds is influenced by:

A$.$ The size of the vessel

B$.$ The flow rate of the gas phase only

C$.$ The properties of the packed solids and fluid flow rates

D$.$ The temperature of the fluid