The relationship between the AAA-yield and BBB-yield for corporate bonds can best be described as which of the following?

A、TheAAA-yieldisalwayslowerthantheBBB-yield;
B、TheAAA-yieldisusuallyhigherthantheBBB-yield,butthatrelationshiphaschangedattimes;
C、ThereisnoobviousrelationshipbetweentheAAA-yieldandtheBBB-yield;
D、TheAAA-yieldisalwayshigherthantheBBB-yield