问题如下：
An excess spread account incorporated into a securitization is designed to limit:
选项：
A.credit risk.
B.extension risk.
C.contraction risk.
解释：
A is correct.
An excess spread account, sometimes called excess interest cash flow, is a form of internal credit enhancement that limits credit risk. It is an amount that can be retained and deposited into a reserve account and that can serve as a first line of protection against losses. An excess spread account does not limit prepayment risk, extension, or contraction.
B和C选项能解释一下吗？
