Film Behind a Double Slit Is Exposed to Light in the Following

Film behind a double slit is exposed to light in the following way: First one slit is opened and light is allowed to go through that slit for time $\Delta$ t.Then it is closed and the other slit is opened and light is allowed to go through that slit for the same time $\Delta$ t.When the film is developed the pattern will be

A) one single slit pattern.
B) two superimposed single slit patterns,their centers displaced from each other by the distance between the two slits.
C) one double slit pattern.
D) two double slit patterns,their centers displaced from each other by the distance between the two slits.
E) random darkening of the film.(no pattern at all)