The lines that must be parallel in each figure includes
18. lines x and y
20. lines l, m, and n then a and b
21. a and b
How to identify the parallel lines
Parallel lines have a special property when intersected by a transversal.
If angles formed by the transversal and the 2 or more lines are equal then the lines are parallel
18. line x and line y formed equal intersecting angle with line z
19. None of the angles are common to a transversal line hence the angle must not be equal
20. lines l and n have equal angles through b and line n and m share equal angles through a. Hence lines l, m and n are parallel.
Through line n, we can say that line a and b are parallel
21. lines a and b share equal intersecting angle through line n. No other relationship proves any other lines are parallel here