Final answer:
Topographic lines point upstream due to gravity's influence, as water flows from higher to lower gravitational potential energy. This is indicated by the V-shaped contour lines on topographic maps, with the narrow end pointing toward higher elevations.
Step-by-step explanation:
The rule of thumb that topographic lines always point upstream refers to the V-shaped patterns that the contour lines form on a topographic map. When a contour line crosses a stream or river, the V-shape will point in the opposite direction of water flow, which is upstream. This is due to the fact that the water erodes the land creating the V-shape, and the narrow end of the V indicates the higher elevation where the water comes from. The correct answer explaining why topographic lines always point upstream is C) Considering gravity's influence, as the stream water flows from areas of higher gravitational potential energy to lower, influenced by gravity's pull.
Contour lines are useful for learning about gravitational potential in different areas because they represent lines of equal gravitational potential energy per unit mass or gravitational equipotential. As the spacing between contour lines decreases, the steepness or gradient of the terrain increases.
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