答案：C

解析：

Let's examine the slope and y-intercept of both graphs.

Both equations are written in slope-intercept form: y=mx+b, where m is the graph's slope and b is the graph's y-intercept.

The graph of the first equation, y=x+a, has a slope of 1 and its y-intercept is a.

The graph of the second equation, y=-x+a, has a slope of -1. Its y-intercept is also a. Therefore, the y-intercepts are the same; the slope of the first equation's graph is the negative of the reciprocal of the slope of the second equation's graph (the reciprocal of 1/1 is 1/1), which means that the lines are perpendicular.