Which of the following would increase a firm's WACC after tax?

a. A firm invests in an average-risk project using debt, rather than equity financing.

b. A firm issues shares and uses the proceeds to pay off a bank loan.

c. A firm issues bonds and uses the proceeds to repurchase stock.

d. A firm significantly improves its operating cost control to boost profits.

e. A pharmaceutical research company develops a 100% effective COVID vaccine, which reduces its systematic risk in the market.