What is the minimum total dry powder fire extinguisher capacity required for an ADR transport unit with a maximum permissible mass between 3.5 tonnes and 7.5 tonnes?

1. 8 kg, with at least one extinguisher having a minimum capacity of 6 kg
2. 12 kg, with at least one extinguisher having a minimum capacity of 6 kg
3. 4 kg, with at least one extinguisher having a minimum capacity of 2 kg
4. 6 kg, with at least one extinguisher having a minimum capacity of 2 kg