What is the minimum total capacity of dry powder fire extinguishers required on an ADR transport unit with a maximum permissible mass of not more than 3.5 tonnes?

1. 4 kg, with at least one extinguisher having a minimum capacity of 2 kg
2. 2 kg, which is the only extinguisher required
3. 8 kg, with at least one extinguisher having a minimum capacity of 6 kg
4. 6 kg, with at least one extinguisher having a minimum capacity of 2 kg