PA Mining Law - Sections 404-406 (2) Practice test questions for the PA Mine Officials' Exam Progress Indicator: Question 2 of 16 2.  Carbon Monoxide is explosive and flammable.  It’s explosive range in normal air is __________ to __________ percent. Choices: 12.0 to 75.0 13.0 to 76.2 12.5 to 74.2 12.5 to 72.4