Exercise 20 Page 525

It sure looks like $a,$ and $b$ are parallel as do $c$ and $d.$ However, we cannot be sure of this. For example, if we are going to be sure that $c$ and $d$ are parallel, they would have to be perpendicular to the same line. We would need to know that they are perpendicular to $a$ or $b.$

The same reasoning applies to determining if $a$ and $b$ are parallel. But we do not know this. Since we have no information about the unknown angles, the figure could be as follows: