A double slit experiment, carried out with light of wavelength $600 \text{ nm}$, produces fringes on a screen. The measured fringe separation is $1.0 \text{ mm}$. If the gap between the double slits and the viewing screen is increased by $2.0 \text{ m}$, the fringe separation rises to $3.0 \text{ mm}$. What is the separation of the double slits that gives rise to the fringes?
- A$0.4 \text{ mm}$
- B$0.6 \text{ mm}$
- C$0.9 \text{ mm}$
- D$1.2 \text{ mm}$