The angle between two rays in 3D space This is a problem from Mathematics GRE Subject Test - #42.

In the xyz-space, what is the degree measure of the angle between the rays $z= (x>=0), y=0$ and $z=  (y>=0), x=0$?
a)0; b)30; c)45; d)60; e)90

My Attempt at a Solution
Because the first set of rays are always along the line y=0, they must be spread out on solely the x-z plane, in the direction of the positive x-axis.
Similarly, the second set of rays would be on the y-z plane, and in the direction of the positive y axis. So I figured that because the rays are on perpendicular planes, they should have an angle of 90 degrees.
Sorry if this is drastically wrong, I'm at a loss where to proceed. I'm not totally even sure what topic to tag this under. Any help is much appreciated. Thanks

The rays are spanned by the vectors (1,0,1) and (0,1,1). The general answer to the angle between two vectors is give by the dot product formula:
$a\cdot b = \|a\|\, \|b\|\cos(\theta)$
In this case, $\|a\| = \|b\| = \sqrt{2}$ and $(1,0,1) \cdot (0,1,1)=1$. So $\cos(\theta)=1/2$ or $\theta=\arccos(1/2)=60^\circ$
