<p>In the xy-plane, line m is the reflection of line l across the x-axis. If the intersection of lines l and m is the point (r, s), which of the following must be true?</p>
<p>(A) r = 0
(B) s = 0
(C) r = s
(D) r = -s
(E) rs = -1</p>
<hr>
<p>Answer is B. </p>
<p>CB gives this explanation: If you draw line L so that it crosses the x-axis at (-2, 0), you will see that the point of intersection of lines L and m is also the point (-2, 0). Similarly, if you draw line L so that it crosses the x-axis at (5, 0), you will see that the point of intersection is also (5, 0). In fact, the point (r, s) must be on the x-axis no matter how you draw line L. This means that r can have any value, but s has only one possible value, 0. Answer choices (A), (C), and (D) will be true if line L goes through the origin, but they do not qualify as statements that must be true. Answer choice (E) cannot be true, since s = 0, and therefore rs = 0.</p>