为自己加油

"On the left side of the isolation valve, the APU bleed air duct joins the left side crossover duct," In the above sentence,what does "isolation valve", "crossover duct" mean?

A.隔离活门，总管;
B.控制活门，交输管;
C.隔离活门， 交输管;
D.双向活门，交输管;