Why do we use an after$-$tax figure for cost of debt but not for cost of equity?

Question $17$Answer

a$.$

Zero coupon bonds allow us to not pay taxes

b$.$

Debt is important only when taxes exist

c$.$

Equity does not provide a tax shield

d$.$

Equity can be collateralized and thus taxes are irrelevant

e$.$

Taxes are not important when evaluating the capital structure.