$9.$ The pressure drop in a packed bed is affected by:

A$)$ The size of the bed particles only

B$)$ The viscosity of the gas phase only

C$)$ Both the fluid velocity and the particle size

D$)$ The temperature of the fluid only

$10.$ Fluidization occurs when:

A$)$ The solid particles settle to the bottom of the bed

B$)$ The velocity of the fluid exceeds the settling velocity of the particles

C$)$ The pressure drop is minimized

D$)$ The gas phase is completely absorbed by the liquid

$11.$ In pneumatic conveying, the solid loading ratio is defined as:

A$)$ The mass of solids conveyed to the mass of carrier gas used

B$)$ The velocity of the gas to the velocity of the solids

C$)$ The volume of the conveying pipe to the volume of solids

D$)$ The pressure drop per unit length of the pipe

$12.$ The main advantage of using a trickle bed reactor is:

A$)$ High gas holdup

B$)$ Efficient mixing of the solid phase

C$)$ High mass transfer rates between the gas and liquid phases

D$)$ Low pressure drop