Read the following SAT test question and then click on a button to select your answer.

y=x+a
y=-x+a

In the system of equations above, a is a positive constant. If the system is graphed in the xy-plane, which of the following best describes the two lines?

A.Both equations represent the same line.
B.The lines are parallel.
C.The lines are perpendicular.
D.The lines intersect but are not perpendicular.

来源:可可英语 http://www.kekenet.com/sat/201801/538704.shtml