Explanation
The return system in an air distribution system takes the hot exhaust air from the IT equipment and returns it to the Computer Room Air Conditioner (CRAC) unit. The CRAC unit cools the hot exhaust air and recirculates it back into the data center. The return system typically includes ducts, grills, and other components that allow the hot air to be efficiently and effectively returned to the CRAC unit.
Supply system is responsible for bringing in the cooled and filtered air from the CRAC unit to the data center.
Maintenance system and regulation system are not related to air distribution system in the data center.

Explanation
Fuel availability generally dictates the type of standby generator selected as different types of generators use different types of fuel. For example, gasoline generators are typically used in residential or small commercial applications, while diesel generators are usually used in larger industrial and commercial applications. Propane or natural gas generators may also be used, depending on the availability of fuel sources.

Explanation
A circuit breaker is a device that protects electrical equipment from damage caused by overload or short circuit. It does this by automatically disconnecting the power supply when it detects a surge or drop in voltage, preventing the equipment from being damaged.

Explanation
Voice identification is a physical security identification method that uses the sound of an individual's voice to identify them. This method is often used in combination with other identification methods, such as biometric scans, to provide a more secure form of authentication. Voice identification is used in a variety of situations, such as when accessing a secure facility or when making financial transactions.

Explanation
Ohm () describes the amount of resistance electricity encounters. It is one of the base units of the International System of Units (SI), and is defined as the amount of resistance that a conductor has when a force of one volt is applied across it. Ohms are used to measure the electrical resistance of a circuit, and can be used to determine the power of a circuit or the amount of current flowing through it.