In Young's Double-Slit Experiment, Light Was First Passed Through a Narrow

In Young's double-slit experiment, light was first passed through a narrow single slit before going on to the double-slits. The reason for the first slit was

A) to screen out all wavelengths except those in the visible light spectrum.
B) to reduce the brightness of the light so the interference pattern could be seen.
C) to polarize the light.
D) to ensure a constant phase relationship between the interfering light waves.