Below the ______, the mixture will not burn because it is too lean.

Below the ______, the mixture will not burn because it is too lean.

• A. Upper explosive limit
• B. Lower ignition limit
• C. Flammable range
• D. Lower explosive limit

Answer: (D)

In a fuel‑air mixture, combustion only happens when the fuel concentration falls within a certain range. The key term is the lower explosive limit, which is the minimum amount of fuel in air needed to support a flame. If the mixture is below this limit, it’s too lean to burn—there isn’t enough fuel to sustain flame propagation even with an ignition source. Above the opposite boundary, the upper explosive limit, the mixture becomes too rich to burn because there isn’t enough oxygen for the flame. The band between these two limits is the flammable range, where ignition and sustained burning are possible. So, the statement is completed by the lower explosive limit.

In a fuel‑air mixture, combustion only happens when the fuel concentration falls within a certain range. The key term is the lower explosive limit, which is the minimum amount of fuel in air needed to support a flame. If the mixture is below this limit, it’s too lean to burn—there isn’t enough fuel to sustain flame propagation even with an ignition source. Above the opposite boundary, the upper explosive limit, the mixture becomes too rich to burn because there isn’t enough oxygen for the flame. The band between these two limits is the flammable range, where ignition and sustained burning are possible. So, the statement is completed by the lower explosive limit.