A firm has the following menu of projects, listed in no particular order: project A, with a yield of 12 percent and a cost of $100,000; project B, with a yield of 8 percent and a cost of $200,000; project C, with a yield of 10 percent and a cost of $300,000; and project D, with a yield of 15 percent and a cost of $250,000. How much will this firm borrow and invest when the interest rate is 12 percent? 10 percent? 8 percent?