An oil driller recently issued USD 250 million of fixed-rate debt at 4.0% per year to help fund a new project. It now wants to convert this debt to a floating-rate obligation using a swap. A swap desk analyst for a large investment bank that is a market maker in swaps has identified four firms interested in swapping their debt from floating-rate to fixed-rate. The following table quotes available loan rates for the oil driller and each firm:

| Firm | Fixed-rate (in %) | Floating-rate (in %) |

|--------------|-------------------|-----------------------------|

| Oil driller | 4.0 | 6-month LIBOR + 1.5 |

| Firm A | 3.5 | 6-month LIBOR + 1.0 |

| Firm B | 6.0 | 6-month LIBOR + 3.0 |

| Firm C | 5.5 | 6-month LIBOR + 2.0 |

| Firm D | 4.5 | 6-month LIBOR + 2.5 |

A swap between the oil driller and which firm offers the greatest possible combined benefit?