As shown in fig. 15-20, two parallel beams of A-rays and B-rays, with a distance of D, are obliquely incident on the parallel glass brick placed in the air, when they appear from the glass brick below

If A is purple light, B is red light, and the incident angle is the same, the refraction angle of A is small, and it is easy to see that the distance between the two outgoing rays increases in the drawing, so option A is correct. If A is red light and B is purple light, and the incident angle is the same, the refraction angle of A is larger. However, because the thickness of the glass brick is unknown, it is impossible to determine the positions of the two outgoing rays, and the distance between the outgoing rays may be zero, greater than d or less than d, so option C is correct.