Which of the following conditions will cause a combustible gas indicator to become inoperative or give erroneous readings?

Correct Answer: Option B — hydrocarbon content of the atmosphere is less than the upper explosive limit

Combustible gas indicators rely on a specific oxygen-to-fuel ratio to function. If the hydrocarbon concentration exceeds the upper explosive limit (UEL), the atmosphere is too rich to burn. The sensor will fail to detect the gas correctly, potentially providing a false reading that the space is safe.