What is a contour interval?

a. a loop formed by contour lines that indicates a peak or depression

b. the elevation difference between two adjacent contour lines

c. an imaginary line that joins points of equal elevation

d. the degree of a slope's steepness

B. The elevation difference between two adjacent contour lines

Explanation:

A contour interval is the vertical distance or difference in elevation between contour lines in topographical maps. A topographical map is a type of map that specifies in the large detail and quantitative representation of relief, usually using contour lines, contour intervals and many other methods.  

rosevelt. 1973. "slaves everywhere"

these civilizations were located around fertile rivers, where they could produce an abundance of crops