The path difference between the two waves meeting at point P, where there is a minimum in the interference pattern is : (A) $8\cdot1 \times 10^{-7}\text{ m}$ (B) $7\cdot2 \times 10^{-7}\text{ m}$ (C) $6\cdot5 \times 10^{-7}\text{ m}$ (D) $6\cdot0 \times 10^{-7}\text{ m}$