问题如下：
What is the evidence on whether rating changes are anticipated?
选项：
解释：
The evidence is unclear. Upgrades do not seem to provide significant news to the market. Downgrades can have an impact on stock prices and bond prices,but this may be because ratings are used for many different purposes (e.g., to define the mix of bonds that can be held by a portfolio manager or what bonds are acceptable as collateral).
不懂这个问题和解答，盼复，谢谢
