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Atendiendo a la siguiente instancia de una relación:
| Atrib1 | Atrib2 | Atrib3 | Atrib4 |
| A1 | B1 | C1 | NULL |
| A1 | B1 | C2 | D2 |
| A1 | B2 | C2 | D3 |
| A2 | B2 | C1 | D4 |
Indique TODOS los conjuntos de atributos que podrían ser Clave Primaria.