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

A、The AAA-yield is always lower than the BBB-yield;
B、The AAA-yield is usually higher than the BBB-yield, but that relationship has changed at times;
C、There is no obvious relationship between the AAA-yield and the BBB-yield;
D、The AAA-yield is always higher than the BBB-yield