Answer:
C
Step-by-step explanation:
1) It's done by making use of one postulate, a theorem whose proof is evident. There is one among many, in Euclidean Geometry, about this particular case.
2)According to Plane Intersection Postulate. If two planes intersect each other, then their intersection is a line. If Plane STL and Plane Z would intersect, they would at line XL, not a line segment.
