Final answer:
X-rays traveling in a useful direction exit through the Tube Port. This is part of the X-ray machine where generated X-rays are directed towards the subject for imaging purposes, such as in CT scans.
Step-by-step explanation:
X-rays that are traveling in a useful direction exit via the B. Tube Port. The X-ray tube is designed with the tube port as the exit point for X-rays that are generated when accelerated electrons strike a metallic target inside the tube. These X-rays are then directed towards the patient or object being imaged. After passing through the subject, they are captured by the image receptor system to produce an X-ray image.
In the context of computed tomography (CT), also known as computed axial tomography (CAT), the X-rays pass through the patient over a range of directions, and multiple slices are taken. Complex computer image processing then produces highly-detailed images, which can be further analyzed to construct three-dimensional information. This principle earned G. Hounsfield and A. Cormack the Nobel Prize in Medicine in 1979.