The contour interval is:a) the heavier contour lines with elevations printed on them b) the difference in elevation between two adjacent contours c) the rate at which the land elevation changes a coordinate system with perpendicular lines of uniform spacing

b) the difference in elevation between two adjacent contours

Explanation:

Each contour line corresponds to a particular elevation above some reference, usually sea level. The difference in elevations represented by adjacent contour lines is called the contour interval.