Originally young's double slit experiment was performed with white light in which central maxima for all colours in the fringe pattern overlap each other at centre. Assume that wavelength of blue light is 520 nm and that of red light is 780 nm. The value of x for which xʰⁿ bright red band coincides with (x + 1)ⁿ blue band is
a. 5
b. 4
c. 3
d. 2