Which of the following hazard classes should NOT be placed into a temperature-controlled trailer (one with a heater/air conditioner unit)?

There are special cargo heater rules for loading:
• Class 1 (Explosives).
• Class 2.1 (Flammable Gas).
• Class 3 (Flammable Liquids).
The rules usually forbid use of cargo heaters, including automatic cargo heater/air conditioner units. Unless you have read all the related rules, do not load the above products in a cargo space that has a heater. [Commercial Driver Handbook]