Which of the following agents should be used on Class B and C fires?

The appropriate agent for Class B and C fires is Halon. Class B fires involve flammable liquids such as gasoline, oil, and grease, while Class C fires are related to electrical equipment. Halon is effective in extinguishing these types of fires because it works by interrupting the chemical reactions that occur during combustion.

In the case of Class B fires, Halon can quickly smother the flames without leaving a residue, making it suitable for use on flammable liquids. For Class C fires, Halon's ability to reduce the supply of oxygen is critical since using water or foam could potentially conduct electricity and create additional hazards.

While foam can also be used on Class B fires by forming a barrier to smother the fuel, it is not suitable for electrical fires (Class C) because it could cause electrocution. Similarly, water is ineffective and dangerous for both Class B and Class C fires due to these same reasons. Dry chemicals do offer a wider applicability by being effective on Class B and C fires, but Halon is specifically noted for its superior control and suppression capabilities when dealing with both types.