The correct answer is D) They do not work properly where there is a lack of oxygen. Combustible gas indicators are designed to detect the presence of combustible gases in the atmosphere. However, they require adequate oxygen levels to function properly. In an oxygen-deficient environment, the combustible gas indicator will not provide accurate readings, as the lack of oxygen can impair the device's ability to detect and measure the concentration of combustible gases. This is why answer D is the correct statement. The other answer choices are incorrect: A) They do not detect a lack of oxygen, but rather the presence of combustible gases. B) A single air sample is not adequate to test a tank, as multiple samples are typically required to ensure proper testing. C) Combustible gas indicators are calibrated to measure the percentage of combustible gases in the atmosphere, not the "chance of explosion."