If the distance between the projectors is altered by a movement along X-axis of one projector, A. The length of the air base is increased B. The scale of the model is altered C. y-parallax is not affected D. All the above

The correct answer is D. All the above.

The length of the air base is the distance between the two projectors. When the distance between the projectors is altered, the length of the air base is also altered. This is because the two projectors are now projecting from different points in space, and the distance between the two points is the length of the air base.

The scale of the model is the ratio of the size of the model to the size of the real object. When the distance between the projectors is altered, the scale of the model is also altered. This is because the two projectors are now projecting from different points in space, and the ratio of the distances between the two points and the distances between the two points and the real object is the scale of the model.

Y-parallax is the difference in the apparent position of an object when viewed from two different points. When the distance between the projectors is altered, y-parallax is also altered. This is because the two projectors are now projecting from different points in space, and the difference in the apparent position of an object when viewed from two different points is y-parallax.