Imagine a severe recession hits and the Fed aggressively cuts rates to zero. However, your portfolio of investment-grade corporate bonds actually loses value. How is this possible? Break down the components of the bond's yield for me.

假设经济遭遇严重衰退，美联储紧急降息到零。但你持有的投资级公司债组合反而亏钱了。这怎么可能？给我拆解一下债券收益率的构成。