Question

which phrase identifies the standard difference in elevation
index contour
contour interval
scale bar
topographic symbol

Explanation:

A contour interval is the standard difference in elevation between adjacent contour lines on a topographic map. An index contour is a darker or thicker contour line that is labeled with its elevation. A scale bar shows the relationship between distance on the map and actual distance on the ground. A topographic symbol represents a specific feature (like a building, tree, etc.) on the map. So, the phrase that identifies the standard difference in elevation is "contour interval".

Answer:

B. contour interval