The suction - force principle of operation is a typical characteristic of （）.

A.a compressed packing gland
B.lantern tings between the packing rings
C.a liquid seal
D.the stuffing box gland

A.loss of second stage vacuum
B.contamination of the distillate
C.lower feed inlet temperature
D.contamination of the second stage condenser

A.total head
B.discharge head
C.net positive suction head
D.suction head