A选项是什么意思？
问题如下图：
选项：
A. 
B. 
C. 
D. 
解释：
A collateralizebonobligation (CBO) consists of severtranches of notes from a repackaging of corporate bon, ranging from equity to super-senior. Whiof the following is generally true of these structures? The totyielof all the Ctranches is slightly less ththe unrlying repackagebon to allow the issuer to recover their fees/costs/profits. The super-senior tranche hexpecteloss rate higher ththe junior tranche. The super-senior tranche is typically ratebelow Aansolto boninvestors. The equity tranche es not absorb the first losses of the structure. ANSWER: A In the absenof transaction costs or fees, the yielon the unrlying portfolio shoulequto the weighteaverage of the yiel on the fferent tranches. With costs, however, the Cyielwill slightly less. Otherwise, the super-senior tranche hthe highest cret ratings anthe lowest loss rate of all tranches, anabsorthe last loss on the structure. C为什么不对
A可以理解为overcollateralization吗？
麻烦请讲解一下A，谢谢
