单选题 Rectangle PQRS lies in the xy-coordinate plane so that its sides are not parallel to the axes. What is the product of the slopes of all four Sides of rectangle PQRS?

(A) -2
(B) -1
(C) 0
(D) 1
(E) 2

【正确答案】 D

【答案解析】Because the figure is a rectangle, the adjacent sides are perpendicular. Perpendicular lines have slopes that are negative reciprocals of each other, meaning that the product of their slopes is -1. Because there are four lines and four perpendicular angles, the product of the slopes is (-1)(1)(-1)(1), or 1.