In relation to metal casting defects, match the following:
| Defect | Description |
|---|
| P. Dross | I. Shallow blow found on a flat casting surface |
| Q. Scab | II. Lighter impurities appearing on the top surface of a casting |
| R. Scar | III. A rough, thin layer of metal, protruding above the casting surface, on top of a thin layer of sand |