What causes current to flow in an electrolytic cell? difference in potential between the three half-cells
a nonspontaneous redox reaction
a spontaneous redox reaction
an outside power source

a nonspontaneous redox reaction

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation: