StockCo is considering investing up to $21,000 cash in some subset of the following four projects:
| Cash Required | Net Return | |
| Investment 1 | $7,000 | $19,000 |
| Investment 2 | $9,000 | $21,000 |
| Investment 3 | $6,000 | $14,000 |
| Investment 4 | $5,000 | $10,000 |
Each project is "all or nothing" -- it is not possible to purchase partial shares in a project. It is also not possible to buy multiple shares. How can the firm maximize its total net return?