问题如下：
The most appropriate response to Kowalski’s question regarding credit rating migration is that it has:
选项：
A.a negative impact.
no impact.
a positive impact.
解释：
A is correct. Credit spread migration typically reduces the expected return for two reasons. First, the probabilities for rating changes are not symmetrically distributed around the current rating; they are skewed toward a downgrade rather than an upgrade. Second, the increase in the credit spread is much larger for downgrades than is the decrease in the spread for upgrades.
请问这个结论在讲义哪里提到过？
