单选题 According to the working principles of fire detectors and control circuits, the commonly used fire alarm systems aboard have different types: the relay type system always uses the sensors with contacts; the microcomputer type can adopt the intelligent sensors which provide more accurate alarm signals. Nowadays, the bus type alarm system becomes the developing direction, which uses field bus to connect all alarm sensors. Besides the fire detectors and manual alarm buttons, there is always a central fire control panel in the fire alarm system which is an electric panel controlling all components of the system. The central panel receives information from environmental sensors designed to detect changes associated with fire, monitor their operational integrity and provide for automatic control of equipment, and transmission of information necessary to prepare the facility for fire based on a predetermined sequence. The panel may also supply electrical energy to operate any associated sensor, control, transmitter, or relay. The central panel is often equipped in the wheel house or the fire station小题:Nowadays, the _____type alarm system becomes the developing direction