Explain the most probable reason why the market price of these bonds has declined, even though Clear-Air‘s credit rating has improved.

The information about old and current bond prices is missing, but the answer is probably the same with or without it. The reason why bonds' prices increase or decrease even if their credit rating remains the same is that market rates change. For example, if the market interest rates decrease, the price of bonds will increase. But on the other hand, if the market rates increase, the price of bonds will decrease. Market rates give are determined by averaging the returns of similar investments, and if an investor believes that he could earn more money somewhere else, he will sell the bonds and invest in that other security.