$1-2.(2$ points$)$ Which of the following statements are true of the conservation of mass?

a$.$ The rate of change of mass within a control volume minus the net flow rate in and plus

the net flow rate out of the control surface must equal to zero.

b$.$ The conservation of mass principle is only applicable to closed systems.

c$.$ The conservation of mass principle is true only if the control volume has a fixed volume.

$($d$)$ The conservation of mass principle holds only for incompressible fluids.

d$.$ In a steady flow process, the total rate of mass entering a control volume is equal to the

total rate of mass leaving it$.$