Workers will enter an excavation deeper than 4 feet next to an old fuel tank where a hazardous atmosphere could exist. What is the BEST practice before and during entry?

Giải thích

Where a hazardous atmosphere could reasonably exist in an excavation over 4 feet deep, the air must be tested before entry and controls such as ventilation provided as needed, with retesting as conditions change. Option a exposes workers before detection. Option c assumes conditions that may not exist. Option d waits until harm may already have occurred. Atmospheric testing detects oxygen deficiency or toxic or flammable gas before it injures anyone.