生成这个题目的讲解---**Question Stem:** 如图，点P是直线a外一点，PB ⊥ a，点A, B, C, D都在直线a上，下列线段中最短的是 ( ) (As shown in the figure, point P is a point outside line a, PB ⊥ a, points A, B, C, D are all on line a. Which of the following line segments is the shortest?) **Options:** A. PA B. PB C. PC D. PD **Chart/Diagram Description:** * **Type:** Geometric figure illustrating a point outside a line and line segments connecting it to points on the line. * **Main Elements:** * **Line 'a':** A horizontal straight line. * **Points on Line 'a':** Four distinct points, A, B, C, and D, are located on line 'a' from left to right. * **Point P:** A point located above line 'a'. * **Line Segments:** * Line segment PB is drawn from point P to point B on line 'a'. This segment is explicitly stated to be perpendicular to line 'a' (PB ⊥ a), indicating that it forms a right angle with line 'a' at point B. * Line segment PA is drawn from point P to point A on line 'a'. * Line segment PC is drawn from point P to point C on line 'a'. * Line segment PD is drawn from point P to point D on line 'a'. * **Relative Position:** Point B is the foot of the perpendicular from P to line 'a'. Point A is to the left of B. Points C and D are to the right of B, with C being closer to B than D.