Here is a "stack" of X-ray images.
You can see them one by one by clicking and dragging the button in the Slider Bar or by clicking the right and left arrows next to the Slider Bar.
Watch what happens to the numbers in the upper left corner.
Use the Zoom In button to look at the image more closely. Use the Pan button to move the zoomed-in image around. Use the Zoom Out button to reduce the magnification of the image (as far as the original detail).
How many images are in the stack?
To answer the question, fill in the blank below, and click Submit.

Hint: If some of the images are hard to see, click the Contrast button, and adjust the Minimum, Maximum, Brightness, and Contrast sliders until you're satisfied with the image quality.