=t Equivalently, (x,y,z)=(3,2,1)+t(3,1,0) So its direction vector is d=(3,1,0). Compare with the z-axis direction vector k=(0,0,1) : dâ‹…k=3â‹…0+1â‹…0+0â‹…1=0. A zero dot product means the two lines are perpendicular. Since it's neither purely along x nor y, but its direction is orthogonal to the z-axis, the correct choice is