单选题Centrifugal pumps have what advantage(s) over reciprocating pumps?（）.A They are less expensiveB They are smaller for equivalent pumping abilityC They pump more cargo in less timeD All of the above

What is the function of wearing rings found on some centrifugal pumps ________．

A.Absorb erosion of high velocity discharge stream

B.Seal pump shaft against entry of air

C.Isolate the outlet side from the inlet side

D.Dampen the turbulent discharge flow

Centrifugal pumps have what advantage（s）over reciprocating pumps ________．

A.They are less expensive

B.They are smaller for equivalent pumping ability

C.They pump more cargo in less time

D.All of the above

Robots have one advantage over humans-they never ____ lack of sleep or food.

A.benefit from

B.suffer from

C.stem from

D.result from

Which pump must always be primed ________．

A.Centrifugal pump

B.Reciprocating pump

C.Rotary pump

D.All of the above

What is/are the advantage（s）of cranes over conventional cargo booms ________．

A.Cranes are able to pick up and drop loads over a greater spotting area

B.Increased safety because the deck is clear of running and standing rigging

C.Simplicity of operation of the crane by its operator

D.All of the above

What is an advantage of a gate valve over a butterfly valve ________．

A.Less frequent maintenance

B.Faster operation

C.Cheaper

D.More compact

Paul: Bob, ________________?

Bob: Oh, that's my father! And beside him is my mother.

A、what is the person over there

B、who's talking over there

C、what are they doing

D、which is that