For two lines to be parallel, their slopes must be equal. This is the key characteristic of parallel lines in coordinate geometry: they maintain the same slope and therefore never intersect.

Correct answer:

B. Their slopes are equal.

Would you like further clarification on this topic? Here are 5 related questions for you to explore:

1. How do you calculate the slope of a line given two points on the line?
2. What are the properties of perpendicular lines in terms of slopes?
3. How can you determine if two lines are parallel using their equations?
4. What happens to the slopes of lines when the lines are vertical or horizontal?
5. How do slopes change if the angle between two lines increases or decreases?

Tip: When working with line equations in the slope-intercept form $y = mx + b$, the slope is represented by $m$. For parallel lines, the slopes ($m$) must be the same.