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A mixture of 1.5% gas and 98.5% air is significant because the percentages indicate the composition of the mixture in terms of its flammability and ability to sustain combustion. A gas concentration of 1.5% is below the lower flammability limit for most gases, meaning that there isn't enough gas present for combustion to occur.

In this instance, the gas does not have sufficient concentration to mix with air in a way that supports a continuous combustion reaction. Therefore, even if an ignition source were to be present, the mixture would not burn. The correct choice emphasizes that without achieving the appropriate concentration of the combustible gas, the mixture remains non-flammable.

This understanding of lower flammability limits is crucial for safety regulations and fire prevention strategies in environments where gases are utilized. The absence of sufficient gas in the mixture is why it cannot ignite or lead to burning or exploding, reinforcing the choice that it will not burn.