In a Young's double slit experiment, the separation between the two slits is `0.9 mm` and the fringes are observed one metre away. If it produces the second dark fringe at a distance of `1 mm` from the central fringe, the wavelength of the monochromatic source of light used is

Options

A. `6 xx 10^-7 m`
B. `6 xx 10^-6 m`
C. `6 xx 10^-8 m`
D. `7 xx 10^-7 m`.