...the sun threw a bright dust-laden bar through one of the side windows, and in and out of the beam flies shot like rushing stars.
The sun square was on the floor now, and the flies whipped through it like sparks.
Both men glanced up, for the rectangle of sunshine in the doorway was cut off. A girl was standing there looking in.
Of the following choices, what is the MOST significant effect of Steinbeck's references to light and dark in the excerpts above?