A two-year zero-coupon bond issued by corporate XYZ is currently rated A. One year from
now XYZ is expected to remain at A with 85% probability, upgraded to AA with 5%
probability, and downgraded to BBB with 10% probability. The risk free rate is flat at 4%.
The credit spreads are flat at 40, 80, and 150 basis points for AA, A, and BBB rated issuers,
respectively. All rates are compounded annually. Estimate the expected value of the
zero-coupon bond one year from now (for USD 100 face amount).

这个credit spread不是说明公司债券和国债的收益率差吗，虽然知道是要算期望，但是不懂这个credit spread怎么运用，求解，多谢！！！