Pay attention to the problem. A and B are two points outside the straight line CD, that is to say, A and B are definitely not on a straight line. The perpendicular passing through point B passes through point A, which proves that the perpendicular passing through point B is perpendicular to the straight line CD. Your question has already given the answer (passing through point B to make the perpendicular of CD), and points C and D are points on the straight line, so point B is the perpendicular of points C and D..

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