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.

an angle + its complement = 90 degrees

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