The formula for calculating lower limit of explosibility of gas mixture [X] = ?

A. $$\left[ {\frac{{100}}{{\frac{{{P_1}}}{{{N_1}}} - \frac{{{P_2}}}{{{N_2}}} - \frac{{{P_3}}}{{{N_3}}}}}} \right]\% $$

B. $$\left[ {\frac{{100}}{{\frac{{{P_1}}}{{{N_1}}} + \frac{{{P_2}}}{{{N_2}}} + \frac{{{P_3}}}{{{N_3}}}}}} \right]\% $$

C. $$\left[ {\frac{{100}}{{\frac{{{N_1}}}{{{P_1}}} - \frac{{{N_2}}}{{{P_2}}} - \frac{{{N_3}}}{{{P_3}}}}}} \right]\% $$

D. $$\left[ {\frac{{100}}{{\frac{{{P_1}}}{{{N_1}}} + \frac{{{N_2}}}{{{P_2}}} + \frac{{{N_3}}}{{{P_3}}}}}} \right]\% $$

Answer: Option B