9. On disassembling a centrifugal pump for overhaul, the third engineer reports holes drilled axially in the back side of a single suction closed impeller. The reason for the these holes is which of the following?

Axial holes in a single-suction closed impeller's back shroud equalize pressure, reducing axial thrust on the rotor and thrust bearing. These balance holes allow liquid flow from the high-pressure front side to the low-pressure back side, minimizing pressure difference and protecting the thrust bearing.