Colleen replicates an X-ray machine by covering an open shoebox with a screen and positioning a square cardboard piece in the center. She uses sand to simulate X-ray radiation, sprinkling it over the screen. Upon removing the screen, she observes an outline of the square cardboard. What does the cardboard symbolize in her simulation?
a. the veins that look white in an X-ray
b. the person being scanned with X-rays
c. the areas that remain black on the X-ray
d. the bones that look white in an X-ray