According to Article 56 of the Implementation Regulations of the PRC Road Traffic Safety Law, what restriction applies to trailer towing by a Class B2 heavy freight truck?

1. It may tow one trailer only, and the trailer's rated load must not exceed the towing truck's own rated load
2. It may tow two full trailers of any weight provided the speed stays below 40 km/h
3. It may tow one trailer only, and that trailer's total mass must not exceed 700 kg
4. It may tow a semi-trailer of up to 20 tonnes without holding a Class A2 licence