问题如下：
Which of the following type of debt obligation most
likely protects bondholders when the assets serving as collateral are
non-performing?
选项：
A.Covered bonds.
B.Collateral trust bonds.
Mortgage-backed securities.
解释：
A covered bond is
a debt obligation backed by a segregated pool of assets called a “cover pool.”
When the assets that are included in the cover pool become nonperforming (i.e.,
the assets are not generating the promised cash flows), the issuer must replace
them with performing assets.
这里non-performing是什么意思？
