A Frangible Bulb Heat Detector Is Activated When

A frangible bulb heat detector is activated when:

A) a beam of light is reflected onto a photoelectric cell to complete a circuit.
B) ionized molecules make air less conductive, causing a decrease in current.
C) a chemical pellet melts, releasing a pressurized plunger to complete a circuit.
D) a small glass vial breaks when heated, allowing electrical contacts to complete a circuit.