Answer:
Option A. Increase in the yield to maturity of the firm's outstanding debt
&
Option D. Decrease in the firm's tax rate
Step-by-step explanation:
The reason is that the increase in yield will result in increase in investor's required return which will increase the after tax cost of debt. Similarly the decrease in tax will also increase the cost of debt to the company because the (1-tax rate) factor is always used in calculating the cost of debt to company.