Which of the following would increase a firm's WACC after tax? O a. A firm significantly improves its operating cost control to boost profits. O b. A firm issues shares and uses the proceeds to pay off a bank loan. O C. A firm invests in an average-risk project using debt, rather than equity financing. O d. A firm issues bonds and uses the proceeds to repurchase stock. O e. A pharmaceutical research company develops a 100% effective COVID vaccine, which reduces its systematic risk in the market