Which statements about extradition are correct?

A、It’ssuchalegalmechanismasthatastatesurrendertoanotherstateoffenderswhocommittedagainstthelawsoftherequestingstateandfleetaboard.;
B、Extraditionisusuallyexercisedundercertaintreatiseorreciprocitypractice.;
C、It’sanobligationofstatetoextraditeorprosecutetheoffenderswhohavecommittedinternationalcrimes.;
D、Therationalebehindthelawandpracticeofextraditionisadesirenottoallowseriouscrimestogounpunished.