Answer:
11. always
12. always
13. always
14. never
15. never
16. always
17. always
18. sometimes
19. sometimes
20. never
Step-by-step explanation:
11. A line can be defined by at least two points
12. A plane can be defined by at least three points
13. If two planes intersect, then they form a line at the intersection.
14. If two lines intersect, then they form a point at the intersection (not a segment).
15. A line does not have any endpoints (infinite in both ways).
16. If two planes are not parallel they will always intersect.
17. If two lines are not parallel they will always intersect.
18. Two points are sometimes collinear. It depends on where they are.
19. Two points are sometimes coplanar. It depends on where they are.
20. Ray MN and NM are not the same ray because the end points are different.