麻烦写个过程吧

问题如下：

As a result of the credit crunch, a small retail bank wants to better predict and model the likelihood that its larger commercial loans might default. It is developing an internal ratings-based approach to assess its commercial customers. Given this one-year transition matrix, what is the probability that a loan currently rated at B will default over a two-year period?

选项：

A.

17.50%

B.

20.0%

C.

21.1%

D.

23.5%

解释：